System reboots itself when logging on as another user

I am currently operating 2000 XP. I recently downloaded and installed Spybot, Ad Aware SE, and Spywareblaster. When I am signed on, leave long enough where I need to resign to get back on, the system reboots on its own. Could the recent installation of the skyware programs have caused this? I have never come accross this one. Your suggestions for help would be appreciated.