Mcaffee do a free program called "Stinger"
http://mcafee-avert-stinger.en.softonic.com/
Download and run it, or even better as Rictus says, download it, then boot your PC in safe mode and run it. If its a common virus the stinger will clean it off.
If you have mal or spyware ontop of that you'll have a battle on your hands. Personally if you havent ever run any Anti-Virus on your PC then I'd pull the plug on your install and wipe the whole lot and start again as you may find you'll never get rid of it totally. Its a drastic solution so try the above stuff and the advice thats already been posted first.