Just a thought, I came across something similar at my wifes work. People could be on a PC for hours no problem, as soon as the machine was left for a while it would lock up and could only be resolved by turning the PC off and on again.
The solution I came across was this. Check the Screen saver settings. At her work the screen saver was on and the "use password on resume" was checked, so you would need to enter your password to unlock the PC. The only problem was, non of the users have passwords set up. So, they couldn't unlock the Screen Saver.
Have a look at that and see how you get on.
Cheers
Rockape
