if it's a virus or trojan, it could very well screwed up things already beyond fix. But let's not ussume the worst yet. what i would do 1st is to get a copy of cc cleaner
and counterspy >> here
Then bootup to safe mode using the f8 button, you can install these during safemode..then use cc leaner and counterspy. be shure that you have them both to deep cleaning.
After that right click my computer and properties/ then system restore tab..once there turn off the system restore by unchecking the box.
Then reboot, and turn back on system restore again, if it does'nt fix the problem then you might have to do a fresh install of the OS.