PC reboots by itself at any time

I am running Windows ME with an AMD 1gig & 512mb RAM. I am having a problem with my PC rebooting at anytime, for no reason. I built the PC in April of 2001 using all new componets. This has just started about 2 weeks ago. I have not loaded any new software and I have scanned for viruses (McAfee). The PC will reboot if I’m on the Internet or not. One day I clicked an Internet link and it shut down and rebooted. No errors, just a reboot. Another time I was playing a PC game, not an Internet game, and it rebooted.

Any ideas on what could be causing this?