Help! How can I remove security tool virus ?

Hello to the Geekpolice, Here goes for my first post. The machine in question has xp (homeuser edition) It has been dormant for a year whilst my son went a travelling. He updated to avg 8.5 today and somehow got the "security tool" program disrupting most of the windows activity. It seems to block you opening anything that might stop it's disruption. i.e. taskmanager to view and stop the rogue processes.Any tool that you download to rid it will not open. I ran spyware doctor which picked up the virus and I thought that would be it but upon reboot it just comes back on.It won't even let me open up the winlogon file that I downloaded, so I can't let anyone analyse that. I would be very grateful for any help you can give me. Many thanks, Phil.

Thanks for the quick response. My son seems to have solved the problem. He booted up in safe mode, he then ran spyware doctor which has spotted and removed the Security tool, and the system seems to be running fine again, but, I will be sure to direct people to this site for any problems, thanks again.
Fliptat.