[David F]

Check msconfig for any dodgy start up files.Note location of registry entry of these files.
Download and install avast during the install it will offer you the option of a boot time scan select yes and reboot.
The scan will start before most windows componants are running allowing the files to be deleted. Wait till it finds a virus and select delete all.Once pc has booted check msconfig for dodgy start up items .
If still present goto start run select regedit and run it. Browse to the location of said dodgy start up items in reg and delete the keys.(also Untick from start up). If its a file name then run a search though the registry for the rogue file ie illl.exe etc. Uninstall and reinstall avast selecting boot time scan again run full scan . This has often rescued me from reformats but it does depend if any of the virus's were rootkits or not