Can you boot up in Safe Mode? http://windows.microsoft.com/en-US/windows-vista/Start-your-computer-in-safe-mode
Might be a good time to do a serious virus/Trojan/malware scan.
AVG Rescue CD is a bootable CD that you can create which also has a AVG AV scanner.
It allows you to run an AV scan while bypassing a possibly infected operating system.http://www.avg.com/us-en/226386