QI let my antivirus software expire and I got hit by the Bagle virus. Following AOL's advice, I uninstalled AOL 9.0, loaded a new antivirus program and reinstalled 9.0, but I'm still having problems.

AIt's tough to hit the brakes after you've gone over the cliff.

Antivirus programs are good at preventing known threats from entering your machine. If, however, a virus has already executed its mischief, an antivirus tool's job becomes vastly harder -- viruses routinely leave bits and pieces of code all over the hard drive.

As a result, antivirus programs can do only a mediocre job of killing viruses once they have contaminated a system. Some viruses can even reinstall themselves after they've been detected and deleted.

In cases like this, the only sure recovery method is to reformat the hard drive, reload the operating system, set up a firewall, get all of Microsoft's security patches -- and then keep an antivirus utility active and up to date.

Am I safer against viruses if I use a Web-mail service like Hotmail?

Microsoft's free Hotmail service scans incoming mail for viruses before delivering it, but that doesn't make it completely safe. Hotmail's virus definitions won't be any more current than those in an up-to-date antivirus package on your desktop; brand-new viruses could still sneak by.

The single best defense against viruses is common-sense skepticism: If the attachment in your e-mail isn't one you were expecting to get, don't open it. The second-best defense is to read your mail in text-only mode, which blocks the tricks used to hide viruses in HTML-formatted messages. If you use Outlook Express -- by far the most susceptible to these tactics -- select Options from its Tools menu, click the Read tab and check "read all messages in plain text."

-- John Gilroy

John Gilroy of Item Inc. is heard on WAMU's "The Computer Guys" at noon on the first Tuesday of the month. Send your questions to him in care of The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or via e-mail to jgilroy@iteminc.com.