PC Restarts Itself Without Any Warning

Hi,

Starting last week, my PC (bought early this year) somehow restarts itself without any warning, unlike the Blaster worm. The screen just turns black, like when you press the reset button.  I happens mostly when I am watching DiVx and XviD videos using Windows Media Player, randomly. Sometimes a few hours after watching, sometimes soon after I open the file.  One time it happened just after I logged in into my Windows XP.  It didn't happen when I left my PC on without running program, nor when running Media Player for running music for a few days.

At first I thought my power supply is the problem.  Then I used ASUS Probe to log the voltages supplied and found that there was no drop in voltages at all.  From the log, I also found that the temperature of the processor was well within the treshold.  I considered some conflicting driver problem, so I updated my video card driver.  Virus scan with latest virus definition found no virus.  I have also reinstalled my Windows XP, but it still happens.

My PC spec:
ASUS P4C800 Deluxe
1GB Kingston HyperX
Sapphire Radeon 9600 Atlantis 128 MB
Maxtor SATA 120 GB
TDK Cyclone CD-RW
MSI DVD
Power Supply 450 W

Software installed:
Windows XP (up-to-date)
Latest Radeon driver
DivX 5.2
XviD 1.0.1
PowerDVD


Any help would be appreciated. Thanks