1) What operating system?
2) Go to Start, Run, type "Msconfig", press enter, go to the startup tab, list everything that's in there.
3) If on Windows XP, can you open Task Manager (Right click taskbar, select Task Manager) and see what takes over all your system resources at that point?
4) If none of the above can help us, give us a list of your running processes from Task Manager.
5) What virus checker did you use?