I strongly suggest that you:

Turn off your computer.
Unplug any network cable.
Find a friend with a computer and licensed anti-virus software.
Have the friend create an emergency boot disk/cd using the very latest virus definitions with his anti-virus software.
Boot your PC using the emergency disk and do a complete scan/clean of your entire system.

Please note, some especially vicious ad-ware type stuff may even require complete wipe of the system as suggested by other posters.

Peski