|Did you recently install any programs? If so then maybe its that program which has given you the virus.|
You can try doing a system restore.
To do this go Start > Then type in the search box System Restore > Then choose a date before you got that virus and hit next, you will then be prompted to restart your PC.
You can also boot into safe mode.
To this just restart your PC and just keep tapping F8 on your keyboard.
You will then be shown an Advanced option screen, hit enter on safe mode.
Run your anti-virus like that, it should work.
Get back to me if you have further problems.