I strongly recommend using the free kaspersky rescue cd. Alternatively, if all is lost, boot in on a linux or any unix-shell os from a removal media system ie, cd, dvd, usb etc and back up all the important files that You want to keep onto another or the same removal media system. After that, best thing is to reinstall the OS. You can download a trial version of windows and use the sticker code on You machine to activate this version of windows.
I hope this helps, please keep me posted.
Going by the results from AV comparatives, give avira a try.