You've been "had".
Unless you're very lucky, there's no way out other than to wipe the hard drive completely and start again.
If you have any personal files etc that you don't want to lose, you could try copying them onto a memory stick first - if the nasty will let you.
I'd try running the Windows setup CD, to see if it will repair the system (worth a try). I wouldn't be hopeful, though.
If that doesn't work, wipe the HD to remove all traces of the infection, then reinstall from scratch.