Too often, security programs significantly slow down the computer, causing lags in booting up the machine, launching programs and receiving email. Not only that, they can be incredibly annoying, popping up frequent messages or asking questions in techie lingo.

The main screen of the 2009 version of Norton Internet Security is streamlined and even includes a gauge to show much drain the security program is placing on the computer.

Now, Symantec (SYMC) has decided to radically rewrite its main security suite for Windows to directly address these problems. And in my tests, this new product, Norton Internet Security 2009, largely succeeded. It isn’t perfect, but it is the fastest, simplest and least obtrusive security suite I have ever used.

Being quick and quiet is great, but, of course, a security product also has to be effective against the vast number of viruses, spyware programs and other malicious attacks aimed at Windows. I don’t have a security lab in which to test such effectiveness. But PC Magazine does, and the magazine called the new Norton suite’s spyware and virus protection “extremely effective.” The magazine’s tests are described at http://www.pcmag.com/article2/0,2817,2330024,00.asp.

However, I wasn’t impressed with Norton’s optional antispam feature, which caused the only significant problem I ran into in my testing.

The new suite costs $70, and can be purchased at symantec.com and elsewhere. For that price, you get to install it on three PCs and you get a one-year subscription to its updates, which cost $60 thereafter.

Symantec is now including free support, even over the phone, though in my tests this support proved lousy. Norton Internet Security 2009 works only on Windows XP and Windows Vista.

I tested the new security suite on a Dell (DELL) desktop running Vista and on a Macintosh laptop running Windows XP as a “virtual machine,” meaning Windows and Windows programs ran simultaneously with the Mac operating system. Symantec assured me the program would run properly in this latter setup, and I had run the previous version of Norton successfully in the same environment.

The first thing you notice about NIS 2009 is the fast and simple installation. The process took under two minutes on each of my test machines.

File scanning is also much faster, partly because the new suite has a feature called Insight that allows it to skip the scanning and rescanning of many of your files. Insight gathers information about your installed programs and compares them against a list of programs Symantec knows are “trusted,” through either its own research or through scanning results voluntarily submitted by other users. These trusted programs are then exempted from future scans, saving a ton of time.

The new Norton suite has a feature called Insight which calculates how many of your programs it knows are trusted, and therefore needn’t be scanned repeatedly, saving time.

For instance, on my Dell running Vista, an immediate complete scan done before Insight analyzed the computer took more than an hour. But after Insight determined that over 70% of my programs were trusted, complete scans took 10 minutes or less. Of course, your data files, like Word documents and emails, still must be scanned, because they are typically unique.

To minimize the impact on users, the new Norton does scanning and other tasks only when it detects that the computer has been idle for at least 10 minutes. And new virus definitions trickle into your computer invisibly, in the background, rather than all at once in a major process.

Symantec is so certain that its product has a low impact on PC performance that it built a gauge into its new streamlined main screen that shows its drain on the main processor, or CPU.

NIS 2009 is also far less annoying than other suites I’ve used. In normal operation, it notifies you only when it has completed a background task or if there’s a threat or a repair that requires a user action, such as a reboot. And there’s an even quieter optional “silent mode” that can turn off nearly every nonurgent activity of Norton for up to six hours. Silent Mode is automatically activated during full-screen activities, such as playing games or watching movies.

The new suite has a bunch of other features, including a browser toolbar for Internet Explorer and Firefox that warns against fake and malicious Web pages, and that can securely enter your passwords and other information on Web sites. It also has an antispam feature for Microsoft (MSFT) Outlook and Outlook Express.

This antispam feature, which is off by default, was rated as weak by PC Magazine and, in my tests, it caused both Norton and Outlook Express to crash repeatedly in Windows XP running on my Mac. Symantec suspects this is a problem particular to running Windows the way I was on the Mac. After I reinstalled Norton and turned off the feature, all was well again.

But Symantec’s free tech-support service was not only unable to diagnose the problem, it didn’t even know the difference between Outlook and Outlook Express.

Despite this one glitch, I can recommend Norton Internet Security 2009 as a good way to protect your Windows computer with minimal impact on your time and attention.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.

Starting this week, just such a smarter navigation box is hitting the market. Called the Dash Express, this $400 product looks a lot like units from better-known firms such as Garmin (GRMN) and Magellan. Like them, it uses GPS satellite signals to locate your car on an easily seen map, and to route you to destinations and places of interest, using both visual and spoken instructions.

But, unlike any other in-car navigation device I’ve seen, each Dash Express, from a Silicon Valley start-up called Dash Navigation, becomes part of a network, connected to the company via the Internet. Each device not only receives and displays information, but transmits it as well, acting as a “probe,” as Dash calls it, to measure local traffic speeds. This information is compiled by the company and then broadcast back to all other Dash units in your area, almost instantly painting streets on your map with color codes to indicate traffic speeds.

I’ve been testing a Dash Express in and around my home base of Washington, D.C., and, while it isn’t perfect, I like it a lot. If the company sells enough units to create a solid network, Dash could radically improve in-car navigation.

That “if” is the big catch with Dash — in order to get its special benefits, enough units must be sold in your city to feed the network with sufficient traffic data. According to the company, for most cities just “several hundred” units would be enough to provide more than half of the significant traffic data it requires for major roads during normal commuting hours.

Meanwhile, Dash, like some of its competitors, makes use of limited traffic data provided by a commercial vendor. This information, which mainly covers major highways, is presented as a dotted line on the Dash maps, to indicate that it may be stale. By contrast, fresh input from Dash’s own network is presented as a solid line.

For the next 30 days, the Dash Express will be available exclusively at Amazon.com (AMZN) for $399.99. You get three free months of Dash’s service, including traffic and other features, such as wireless map updates. After that, the service costs between $9.99 and $12.99 a month. That fee includes the cost of the Internet connection used by the Dash Express, which is achieved using both cellphone and Wi-Fi networks.

To test the Dash, I had to create a tiny two-car network. My colleague Katie Boehret and I each drove the same route in Dash-equipped cars, about 15 minutes apart. The route included everything from the smallest residential streets to large local commuter arteries to the jammed Washington Beltway.

Katie went first, and by the time I retraced her route, my Dash unit’s screen was ablaze with solid-color streets indicating the traffic speeds she had encountered: green for free-flowing traffic, yellow for moderate congestion, red for stop-and-go conditions. Even two-lane local roads, the kind where traffic data are almost never available, were colored in.

Once Dash begins selling, the company won’t rely much on the information provided by a single driver like we did. It will average and weight the information it receives, to eliminate odd results from especially fast or slow drivers, and to emphasize the newest data. Each Dash reading will time out after no more than 25 minutes, turning solid lines into dotted ones as a warning that the information may be old.

I did run into a couple of glitches during the test. For one small road Katie had traveled, I received no Dash data. And on the return trip, Dash tried to route me right into a Beltway traffic jam, even though its screen showed that area in red. The company is working a future feature, called My Route, that would allow savvy drivers to order the device to use the local routes they prefer, to avoid such jams.

Dash Express has a host of other nice features, explained online at dash.net. Instead of giving you one route to your destination, it offers three choices, one of which supposedly incorporates current traffic conditions. It allows you to type in a destination on a personalized MyDash Web page and have that address sent wirelessly to your Dash unit, ready to be selected. You could even have a colleague or friend send you a destination while you are driving, so you don’t have to pull over to type it in or, worse, try typing while driving.

And the Dash also connects to the Internet to perform searches for local businesses, and then routes you there. Plus, you can create your own lists of favorite places and points of interest, or share those created by others and send these to your Dash from the MyDash Web site.

Dash Express finally brings the power of the Internet, and of community information, to auto navigation. If it becomes popular, it could be a big deal.

• Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

The arrival of a large update like this isn’t a sign of trouble, or even unusual. Microsoft has routinely issued these large “service packs” periodically for Windows. And just this week, its competitor, Apple, unleashed an even larger update for its new operating system, Leopard.

Even though they can take a long time to download and install, such updates are generally a good thing for consumers. Microsoft will automatically deliver SP1, as the company calls it, through its normal updating mechanism, built into Windows. The update is free.

However, based on my tests of Vista SP1, I believe that for most average consumers, it will likely be a nonevent, and for others it will be disappointing. Many of its benefits are aimed at corporations and power users, or are under-the-hood fixes that are hard to discern. For mainstream users, it adds no significant, visible features to Vista, and changes little or nothing about the way the operating system looks and works.

Also, SP1 doesn’t resolve some of the most annoying flaws in Vista, including slow start-ups and reboots, and a security system that nags you too much and requires add-on anti-virus software. I guess these problems will either never be fixed fully or will have to wait for SP2.

While Vista SP1 does deliver some performance improvements in certain scenarios, it can actually temporarily degrade performance — including making reboots even slower — because of a quirk in the update process. This slowdown should go away in a few days, the company says.

On balance, the update is probably worth installing, especially since Microsoft will deliver it automatically. But I wouldn’t rush to grab it and I wouldn’t expect much from it. One note: you can’t install SP1 until you have installed a couple of other patches first. These will also be distributed automatically.

I installed Vista SP1 on two computers that had come with the original Vista preinstalled: a 10-month-old Sony Vaio SZ laptop and a two-month-old Dell XPS One desktop. Because the automatic download distribution isn’t yet in place, Microsoft sent me the update on a disk, which also included the prerequisite patches. In each case, the upgrade took a little over an hour and went smoothly. During the process, the computers rebooted multiple times, but it was all automatic and didn’t require user intervention.

After the installation, the computers functioned normally. I tested three of the performance improvements Microsoft claims for SP1. The first involved speeding up the copying of hefty folders containing large numbers of files. On both machines, copying a folder containing over 700 files totaling almost 700 megabytes took less than half as long with SP1 as it had with the original Vista.

I also tested how long it took both machines to awaken from a hibernation or sleep state and be ready for work. For these tests, I began with each machine running Microsoft Word, Microsoft Outlook and the Firefox Web browser, then I forced them into sleep and hibernation mode.

By my definition, “ready for work” means that Vista’s circular delay indicator has gone away, the software that loads at start-up has finished launching and the computer has fully reconnected to its wired or wireless network. On both of my test machines, SP1 improved the recovery time from sleep or hibernation, shaving one to 10 seconds from the procedures.

Microsoft doesn’t claim SP1 will improve the speed of cold starts and reboots under Vista, but I tested these anyway. To my horror, I found that SP1 actually made rebooting — already slower than on comparable Windows XP computers or Macintoshes — even slower.

Microsoft explained that this was due to the fact that installing SP1 erases certain data used by Vista to speed up program launching. It takes the system a few days to build this data back up, the company says. Until then, it says, overall performance, including reboots, can be slower under SP1 than under original Vista.

Microsoft provided me with a method that would rebuild this program-launching data more quickly, at least for the common programs I was using in my tests. Once I followed that method, rebooting time returned to its former state — still too slow for my taste, but at least not worse.

In briefing me on SP1, Microsoft made a big point of saying that great progress had been made in the past year in making Vista work properly with add-on devices, such as printers. I tried my 2003-vintage Hewlett-Packard printer, which hadn’t worked properly with the original Vista. It still didn’t work well with SP1.

So, Vista SP1 is a step forward, at least after a few days of use. But it’s not a big step.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.