can't user switch, or shut down xp

when I go to
start>log off>switch user
or
start>turn off computer>turn off (or restart)

the window then disappears, and NOTHING happens...it's not just hanging...I could leave it indefinitely with no action.

I can't get task manager using ctrl alt delete either...the only solution is to shut down with the power button. I'm just waiting for the day when my wife has a word document unsaved on her profile, and can't get back to save it.

I should mention that this problem only occurs when the PC has been running for awhile. After a reboot, everything works fine.

Any suggestions? thanks in advance!!

 

forgot specs:

Dell 8200
XP SP2
2.0G P4
512 RAM
totally clean...Kaspersky, AdawarePro, Spybot...all up to date...Hijack this scans clean etc

 

Do a system restore.

 

well, this has been a problem for some time...it is not new, so I'd have to go back a long ways. I figured it was high time to address it however.