My PC got a virus

Okay, I need some help. Usually, I am good with computers, but this time I screwed up. I got a virus on my pc. So far, all it seems to do is take FOREVER to log in, give me a black screen after logging in for 5 minutes, remove SP2 (still in windows 7 SP1) and make the CPU usage 100% (I am assuming it underclocked my processor)
AVG free 2013 is REFUSING to scan, and will not let me install anti-virus. My current specs are as follows:

RAM: DDR 2GB
GPU: EVGA 8400 GS
CPU: Athlon 64 3500+ (clock unknown, it is normally 2.2 GHz)
PC: HP pavilion a1130n
OS: windows 7 ultimate SP1
anyone who can help me?

If you have access to another computer MS has a stand alone tool What is Windows Defender Offline? Try booting from that disk. Also did you try booting into safe mode and run AVG?
Joe

Hey Joe,AVG will not run in safe mode and I only have a Vista laptop but it has no CD-ROM/DVD-ROMS. Any other ideas?

The instructions for Windows Defender Offline, tell you how to make set up a Flash Drive you can use.

Thanks, Saltgrass. I will try this ASAP.