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.

You need a good antivirus program, a strong firewall program, an effective antispam program, and a program that specializes in stopping spyware and adware. Or you could just buy an Apple Macintosh, which isn’t significantly affected (so far) by these threats, other than spam email.

But the fastest-growing computer-security problem isn’t viruses or other traditional malicious programs, and it can’t be entirely defeated by using security software or by buying a Mac. It’s called “social engineering,” and it consists of tactics that try to fool users into giving up sensitive financial data that criminals can use to steal their money and even their identities.

Social engineering is a broad term that includes “phishing,” the practice by which crooks create emails and Web sites that look just like legitimate messages and sites from real banks and other financial companies. It’s closely linked to a newly named category of malicious software called Crimeware — programs that help criminals steal your private financial information.

These terms are confusing and overlapping, but the threat is real. Increasingly, common-looking scams are combined with secret installations of software that help criminals spy on you and steal your data.

Here are a few tips to help you avoid these schemes:

1. Don’t trust email from financial institutions. Email is so easily manipulated by crooks that you simply should never, ever consider any email from a financial institution as legitimate. The message may bear a bank’s or a broker’s logo, but you should never respond to such an email, and never click on any link it contains.

There is a very high chance it’s a skillful fraud, and that the link will take you to a clever fake Web site designed to capture passwords and account numbers. The site may also silently install on your PC a program called a key logger, which records everything you type and sends that information back to the crooks.

2. Never respond to unsolicited commercial email, or spam, or even click on a link in an unsolicited commercial email. In the old days, responding to spam just got you on more spam email lists. Today, it might also result in the secret installation of a key logger or other malicious software.

Besides, any company that has to resort to spam as a sales tool isn’t likely to have a very good product to offer. Do you really think that if someone had invented a pill that enlarged penises and breasts, he’d be selling it through spam? He’d have sold it to a big drug company for billions. And nobody in Nigeria needs your bank account to store stolen millions.

Would you buy a stock touted on the street by a complete stranger? If not, why would you buy one touted in a spam email?

The only safe response to spam is to ignore it and delete it.

3. Don’t download or use free software unless you’re sure it’s legitimate. Sites offering free cursors, for instance, can secretly install all sorts of bad stuff on your PC. This is especially true of free security software, which is sometimes just malicious software posing as a security program. If you suddenly see a security program pop up on your PC, don’t trust it.

There are many legitimate free programs, including some good free security programs, like SpyBot or AVG Anti-Virus. But check them out before downloading. Look them up on the CNET or PC Magazine Web sites, which review most software. If they’re not covered there, assume they’re not legitimate. You may pass up some free programs that are real, but it could save you from huge grief.

Earlier, I said that buying Windows security software, or using a Macintosh, can’t automatically protect you from social-engineering schemes, and that’s true. But they can help. An antispyware program can’t prevent you from entering sensitive information on a fake Web site, but it might block the installation and operation of spying software from that site. A Macintosh owner can foolishly give up her bank account number, but most malicious software that crooks try to install won’t work on a Mac.

And there are some new security programs aimed directly at social-engineering scams. McAfee’s Site Advisor program can tell you if a Web site seems bad. A new add-on for the Firefox Web browser, called Shazou, can tell you where a Web site’s server is located. If you think you’re on the Bank of America Web site, but Shazou tells you the server is in Russia, that’s a clue that you’re being scammed. And Symantec plans a new product this fall called Norton Confidential that will tell you if a Web site appears to be a fake. Also, forthcoming new versions of Firefox and of Microsoft’s Internet Explorer browser will have built-in warnings that sites may be fake.

The best defense against social engineering, however, is to be smart and careful.

• Email me at mossberg@wsj.com.