Sunday, November 2, 2003; Page F08

Do I need to update my firewall program every time I update my anti-virus software?

No. Anti-virus and firewall programs go about the objective of protecting your computer in different ways.

An anti-virus utility has to act like a fingerprint expert, scanning incoming files to see if they match the patterns of known viruses. Anti-virus developers constantly update databases of these patterns and set their anti-virus software to download the latest data once a week or so. These automatic updates require a yearly subscription and are worth the cost: Going on the Internet with old virus protection is like asking a detective to identify a set of fingerprints based on five-year-old FBI records -- the bad guys are likely to slip by.

A firewall, meanwhile, is more like window locks in your home. On a computer, these windows are "ports" -- optional channels of communication that can be exploited. A firewall will lock them up, in essence forcing Internet data to enter and exit through the front door.

Firewall updates tend to be limited to patches and feature additions. Anti-virus updates (and firewall bug fixes) are mandatory, but buying new firewall releases is up to your own judgment .

I keep getting "security certificate expired" message from Web sites. What can I do to prevent this?

These certificates, which expire after a set number of years, verify the identities of Web sites. This message could mean something's amiss at a Web site, but the likelier explanation is an obsolete Web browser.

Browsers include their own built-in certificate to set up secure Web transactions. Old versions of Netscape and Internet Explorer had certificates that expired on Dec. 31, 1999. Install a new browser and the problem will go away.

-- 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.