Intermittent logout problem

Every so often my machine enters a state where it appears to be operating normally but I can't log out. Start->Log Off brings up the usual "Are you sure you want to log off?" prompt, but clicking Log Off does absolutely nothing. Shutting down doesn't work either, even when I press the power button, and the screen saver doesn't deploy. Ctrl-Alt-Delete is ignored. Pressing and holding the power button is the only way to turn the machine off. My kids and I share the computer and there's a lot of logging off and on (maybe a dozen times a day), but the problem happens only once or twice a week. Nothing shows up in the event log, and except for not being able to log out or shut down the machine behaves just fine.

System specs:

Gigabye GA-965P-DQ6 Rev 1 mobo F6 BIOS
e6400 @ 2.8GHz (400x7)
2GB XMS2 PC2-6400C4
320GB Raid 0 (hooked up to the ICH8R)
X1900XTX, Catalyst 7.4
Wireless desktop, Bluetooth, xbox 360 gaming receiver, etc.
Windows XP Home SP2b with recent security updates
Active Virus Shield (AOL freeware antivirus)

Any idea what might be causing this? Anything I can do to debug this problem? My BIOS is a bit dated, and I know F6 has problems with the JMicron controller, but I'm in no hurry to update it. Drivers are up-to-date. Any help would be greatly appreciated. Thanks!