Sudden reboot

[xBoom]

When I'm using, idling, chatting or playing a game.. it reboots suddenly..
No BSOD.. And it happened many times. I know it's not a PSU problem becaue I tried two PSU and get the same thing. The first one has a wattage of 430W with 22A on 12V+ rail and the other has a wattage of 400W with 18A on 12V+ rail.
Happens on Vista and XP (as I have two OSes on my computer).

Temperatures are OK (but not too good):
CPU: Idle 43C / MAX 52C
GPU: Idle 41C / MAX 53C

Please help me, it's really annoying..

PS: I scanned for viruses/spyware and I found nothing.

 

[ghost]

Sounds like a HDD problem, I would find out the make and model of the HDD. Then do a google search for the manufacturers site. On there you should be able to download some diagnostic tools.

Usually you can put the software of a floppy disc or bootable CD. Run it at startup and then it should tell you the status of the HDD.