This Jet is a database engine in Windows XP, Vista, and 2000 for use by other programs that you might install, such as Office. It's normally behind the scenes, but a recent zero-day security bug--one that actively attacked before there was a fix--let the bad guys take over vulnerable PCs by targeting a Jet flaw. XP SP2 and Windows 2000 SP4 (and earlier) are at risk; Vista and XP SP3 are safe.

Like many security holes, this one has to do with sending a program more data than it's expecting--basically flooding it with information to cause what's called a buffer overflow. Opening a downloaded or e-mailed booby-trapped .mdb file triggers the attack.

Get the fix via Automatic Updates or from Microsoft.

Installing XP SP3 closes this hole as well--but SP3 caused major headaches for some HP Pavilion and Compaq Presario desktop PCs with AMD processors. Some of those machines with Windows installed a particular way will reboot continuously after SP3 is installed.

If you have a Pavilion or Presario and haven't yet installed SP3, check out a free tool from Jesper Johansson, a former Microsoft employee and a security expert. The tool can evaluate your system and fix the problem before it occurs.

Or if you're already tearing your hair out over an affected PC, head to an HP advisory with instructions on how to break the restart cycle and fix the problem. Also, Microsoft is providing free technical support for XP SP3 installation problems: You can either call 866/234-6020 or browse to this Microsoft support page for XP Home users.