I'll bet it's a startup process that the virus installed in the registry. I've encountered this same problem recently. You could try using a Bit Defender rescue CD (Google it), but first I'd inspect the registry and specific folders in typical areas when viruses meddle with the startup process using a Bart PE CD. If you're a novice then manually sifting through the registry and folders with a Bart PE CD is probably asking way too much; Try creating a Bit Defender rescue CD instead.