Unfortunately, some viruses just don't want to give up. They might even lock you out of your computer completely! How can you possibly fix it?
If your computer won't even start up thanks to a nasty virus, you should try using AVG's Rescue CD. AVG anti-virus is one of my recommended free anti-virus utilities. The Rescue CD is AVG's prescription for really bad infections.
It's a set of tools that doesn't require your operating system to run. It can boot from a CD or USB flash drive and unpack itself to clean your computer of viruses without having to reformat your drive.
When you click the blue "Free Download" button, you'll have 3 options. The first is an ISO file meant to be burned to a CD. The other two options are RAR and ZIP respectively. These should be installed on a USB flash drive.