PC hags when idle

My pc hangs/freezes when left idle for a couple of hours, mouse cant move, ctrl,alt,delete doesnt do anything,
ive done the memory tester with the bootup disk and no errors, it has on occasion just restarted without warning but i unticked auto resart in hope that i would recieve an error message, but nothing, it just hangs.
i really dont have any clue, any help would be great.
my set up is,

P4 3.2
Asus P5VDC-MX mobo
200gb ide drive
1gb ram(2x512mb)
XFX 7800GS Extreme 256mb