It's just trying to trick you into buying the "pro" version of the software (which actually doesn't do anything except pretend to find viruses). Here's the link to the fix (from the page that bipper linked): Remove Vista Defender Pro, removal instructions

It emulates the behaviour of a legitimate program called <b>Windows</b> Defender, which you're likely to see at some point, as long as you have Vista, so don't panic if you see Windows Defender; that one's kosher.