PC Rebooting: Part 2

PROBLEM: PC reboots after I’ve been logged in for at 10minutes. Tried switching hard drives, but the problem still exists. I’ve reinstalled Windows as well as all my other software applications, but it still reboots. I’ve checked my fan, no problem, checked my power supply, no problem. The specs of my machine are as follow:

Intel Celeron 2.4GHz Processor
256MB DDR333 DIMM (2x128MB)
128MB GeForce FX5200 Graphics Card
Western Digital 20GB Hrad Drive (Windows & Software)
Western Digital 60 Hard Drive (Games Drive)
Windows XP Home with Service Pack 2 Installed

The machince is a about 5 years old already and has had numerous upgrades.