Re: I think I may have a virus.
Most likely, it's a virus if your PC keeps rebooting like that. Insert the XP installation CD and enter setup mode. Choose the recovery option and boot to the DOS command line. Execute the command "FIXMBR" and reboot. The "FIXMBR" command re-initializes the MBR with NTLDR.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?