QI keep getting "Messenger Service" pop-ups online, even though I use Firefox. How can I stop them?

AI was floored to get this query last week about these pop-ups -- small gray windows with plain text that can appear on Windows PCs when they go online. They've always been easily defeated by turning on the firewall built into Windows XP or using any other firewall. The Service Pack 2 update to Windows XP also shut off the feature exploited by this intrusive advertising (no relation to instant messaging) a year ago.

And yet this reader did not know this -- he thought his browser choice, along with his antivirus software, should have stopped the problem. Wrong. If you use a Windows machine online, you must put a firewall on it. Without that, messenger pop-ups are the least of your worries; you also risk having your computer taken over by a worm and forced into service as a conduit for spam and worse.

Microsoft provides directions for turning on the firewall in Windows XP at www.microsoft.com/protect/; earlier versions of Windows lack that protection, but you can install such free third-party applications as ZoneLabs' ZoneAlarm (www.zonealarm.com).

Is the Keychain program in Mac OS X safe to store my Web site passwords?

If your account password, which normally also locks and unlocks your Keychain, is obvious enough to be guessed, then the Keychain isn't safe. Also, if you run a version of OS X older than the new Tiger release, you can reset both account and Keychain passwords by booting a Mac off an OS X install CD. (Don't want to upgrade to Tiger? Set a separate Keychain password.)

Either way, though, a successful attack would usually require that the attacker first sit down in front of the Mac. And if strangers can do that in your home, you have far bigger problems than password theft.

-- Rob Pegoraro

Rob Pegoraro attempts to untangle computing conundrums and errant electronics each week. Send questions to The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or rob@twp.com.