I am having problems with Windows XP SP3. Operating System restarts automatically without pressing restart or Automatic Updates. Norton Security Scan-NO VIRUS (SAFE) Lavasoft Adware-Safe, Registry Cleaner-Clean.
What could be the problem and how to solve?

apparently my "Norton Security Online" has been cancelled by At&t. now i need to find a good security software program with virus protection and advanced firewall.
and if i do find one should i Uninstall the one that is cancelled?