There are a few viruses like WinLock, which do similar things, but if you've done a virus check... is your virus checker an updateable one (to make sure it's checking for the most recent viruses)?
Another thing you can do is go to Start -> Run and type in msconfig although I warn you now do not change anything contained here unless you know what you're doing. One of the tabs should say something like "Boot List" or something, which gives you a list of everything that loads when your computer boots up. Check this list, especially programmes at the bottom and look for any suspiciously titled files (as a rule they're easily distinguishable). If you think you've found something that shouldn't be there, you can uncheck it to stop Windows loading it. If your computer works fine once it's been disabled, then you can start work on deleting it.