|If it is a virus you have, I don't think you need to reformat your computer unless it has completely destroyed your OS and you can't do anything.|
Try going into safe mode and running the virus scan from there. (Press F8 when booting your computer)
If for some reason you can't get into safe mode, go to start --> run and type in msconfig . From the first dialog box, select "Diagnostic Startup" or if you are a bit more advanced click "selective startup", and disable everything excep the necessary os items and virus scan.
Once you run the scan and remove any viruses you can then go back into msconfig and select normal startup again. Restart and you should be good to go.
Also, while in diagnostic mode you might want to check for spyware.
hope this helps,