The new aluminum-clad MacBook Air, which I’ve been testing for several days, is billed as the world’s thinnest notebook computer. Its thickest point measures just three-quarters of an inch, which is slimmer than the thinnest point on some other subnotebooks. And it employs some innovative software features, such as fingertip gestures for its touchpad that are similar to those on Apple’s iPhone.

Apple refused to make the most common compromise computer makers employ to create their littlest laptops. Other subnotebooks — a category generally defined as weighing three pounds or less — have screens of just 10 to 12 inches and compressed keyboards. The three-pound MacBook Air, by contrast, features a 13.3-inch display and a full-size keyboard.

It’s impossible to convey in words just how pleasing and surprising this computer feels in the hand. It’s so svelte when closed that it’s a real shock to discover the big screen and keyboard inside.

But there’s a price for this laptop’s daring design: Apple had to give up some features road warriors consider standard in a subnotebook, and certain of these omissions are radical. Chief among them is the lack of a removable battery. So, while the MacBook Air will be a perfect choice for some travelers, I can’t recommend it for all. It really depends on your style of working on the road and what features you value most.

The MacBook Air, which will be available next week, costs $1,800 with an 80-gigabyte hard drive and a generous two gigabytes of memory. A second model, with a faster, cutting-edge, 64-gigabyte, solid-state drive and a slightly speedier processor, costs a whopping $3,100. The $1,800 price for the main model isn’t unusual in subnotebooks, which can easily top $2,000, although some competitors cost less.

In my tests, the MacBook Air’s screen and keyboard were a pleasure to use. The machine felt speedy, even with multiple programs running. And the laptop has the same Leopard operating system, superior built-in software, and paucity of viruses and spyware that I believe generally give the Mac an edge. I was able to install and run Windows XP using the third-party Parallels software.

But then there are those trade-offs. The sealed-in battery means you can’t carry a spare in case you run out of juice, and you have to bring it to a dealer when you need a new one. There’s no built-in DVD drive. The thin case can’t accommodate a larger internal hard disk. And the machine omits many common ports and connectors.

The MacBook Air

There’s no Ethernet jack for wired broadband Internet connections and no dedicated slot for the most common types of external cellphone modems. That means that out of the box, the MacBook Air has only one way to get on the Internet — through its fast, built-in Wi-Fi connection. If you’re out of Wi-Fi range, you’re out of luck, unless you buy an optional, $30 add-on Ethernet connector or a cellphone modem that connects via USB.

In fact, the MacBook Air has only three connectors: a headphone jack, a single USB port and a port for connecting an external monitor.

That single USB port is a problem, because so many peripherals use USB. You can buy a tiny, cheap USB hub that adds three more ports, but that’s yet another item to carry.

The lack of a DVD drive is partly solved by some clever software Apple included that lets you “borrow” the DVD drive on any other Mac or Windows PC on your network, so you can transfer files or install new software from a CD or DVD. This worked fine in my tests, in which I installed several new programs from CDs on remote computers, but it requires disabling third-party firewalls on Windows machines. It also doesn’t work for installing Windows on your Mac, for watching DVDs, or for playing or importing music. For those tasks, you need an external DVD drive. Apple sells one for $99.

In my standard battery test, where I disable all power-saving features, set the screen brightness at maximum, turn on the Wi-Fi and play an endless loop of music, the MacBook Air’s battery lasted 3 hours, 24 minutes. That means you could likely get 4.5 hours in a normal work pattern, almost the five hours Apple claims.

But the MacBook Air has another downside: its screen height. Because of the larger screen, the lid stands higher when opened than on most other subnotebooks. So it isn’t as usable as some competitors when the seat in front of you in coach on a plane is reclined.

If you value thinness, and a large screen and keyboard in a subnotebook, and don’t watch DVDs on planes or require spare batteries, the MacBook Air might be just the ticket. But if you rely on spare batteries, expect the usual array of ports, or like to play DVDs on planes, this isn’t the computer to buy.

• 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.

Just use an Internet-based service so that you can chat from a Web page without having to install any software, which might be blocked by a firewall. I tested two such services: Meebo at www.meebo.com and KoolIM at www.koolim.com. Both are free.

These services let you simultaneously log in to multiple IM accounts — and communicate with people with various services. If you have a friend who uses Yahoo Messenger, for example, and another who likes MSN Messenger, you can chat with either.

Another plus: Meebo and KoolIM are far less vulnerable to viruses than downloadable applications. They’re also more efficient, saving users the hassle of installing multiple programs on a computer. This is especially handy for people with old computers that slow down when running several applications.

Meebo has a well-designed, sleek interface that makes it appealing to even the least tech savvy. From its home page, you simply sign in for different IM services—MSN Messenger, Yahoo Messenger, GTalk (or Jabber) and AIM (or ICQ). Your buddy list will be combined automatically. You don’t have to register, but if you do, you get perks such as a single sign-on for all of your accounts, and the ability to share files, save chat logs and store conversations.

I tried Meebo on my work Windows PC and my iBook at home, and it worked well on both. To start chatting, you just log in to any of the IM services by entering the screen name and password you already have with a service, or by picking a new name, password and services. Your buddy list will appear in a window on the right side of the page, with each name marked by an icon denoting the service the person uses. Once in your buddy list, you can add or delete a contact, message or join a group chat.

The service also offers MeeboMe, a way to chat from your own Web site. Bloggers can use this to start a conversation with site visitors who log in to their Meebo account. It also shows how many users are on the site at any time.

But Meebo, like most such Web services, lacks the ability for audio and video chats — basic features on installed IM software. If you’re used to plugging in a Webcam and headgear to chat with friends, these services might not be for you. Meebo also doesn’t have the standard send button that’s on installed IM software. Instead, you press a key on your keyboard.

Sharing files using Meebo is tricky, too. Only Meebo account holders can send files; although anyone can receive them. When I sent a PDF file to my friend on her installed MSN Messenger, she received a message and a link that rerouted her to Meebo’s home page with no further guidance. When I sent her the same file after she logged in to her MSN using Meebo, she could open the file.

KoolIM also connects users to multiple IM accounts from one Web page. It gives you the option to create an online account, so you don’t need separate log-ins for each IM service you’ll encounter.

You can select an embedded or a pop-up version of the service. The embedded version shows your buddy list and messages as windows tied to a Web page, while the pop-up version lets you move the windows around the screen. You can choose to display all of your buddies or only the ones online. In your buddy-list window you can add and delete names and log in to or out of a service.

I didn’t like having text ads appear at the bottom of the IM window in KoolIM, or the fact that, unlike Meebo, there isn’t a time stamp on messages to tell you when they arrived. It also doesn’t allow you to send files, as Meebo does. And it doesn’t let you save conversations or chat logs. KoolIM’s lack of some of these basic features might make it less appealing to consumers, especially those who ponder swapping it with their desktop software.

I preferred Meebo, but both services are good alternatives to installed IM, if you’re willing to do without audio and video features.

Walt Mossberg is on vacation. Find all my columns and videos online free at the new All Things Digital Web site, http://walt.allthingsd.com.

• Email me at Sarmad.Ali@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.