First of all, boot into safe mode by holding F8 during startup. Out of the list of options choose Safe Mode With Networking, as you'll need internet access to fix this.
Once you're up and running in safe mode, pop over to
HouseCall - Trend Micro USA and run a scan.
Without knowing what you've picked up, it's impossible to give more detailed instructions. If housecall doesn't take care of it, we'll have to figure out exactly what virus we're dealing with before we can go any further.