PC constantly rebooting, virus?

I am having serious trouble with my PC. It just keep rebooting the moment the power is turn ON. On rare occasion, it manage to start-up the Window. One occasion, I managed to download and run Microsoft Malicious Malware Removal Tool, and got rid off 6 infected files. But, the rebooting got more serious…as mention.

I am running on Window XP-Pro, 2002 version, with Service Pack 2. However, my Norton Anti-virus expired Y2003.

Any ideas to fix this problem?

Anthony