There are a lot of viruses out there, but many of them follow the same pattern: they lock you out of your system and only let you visit their site, pretending to be anti-virus software. Here's a trick to squash them.
There's more than one way to kill a virus, but if something's locking you out of computer, the How-To Geek has a great way to trick it into letting your Anti-Malware tool run a scan:
Since most of the fake anti-virus malware needs you to be able to slightly use your PC, the one executable that it won't ever block is "explorer.exe", since they want you to be able to get online and go to their site and pay them-not so easy if you have no Start Menu.
So just rename your favourite anti-malware application to explorer.exe, and you should be able to use it.