Help removing a virus

Hello,
My son was downloading music. He downloaded a virus as well. The only thing it will let me do is go to the Antivirus Live (Antivirus Pro 2009) internet site. I can't access any other web sites. Window secrurity alerts are constantly popping up. How to I restore the factory settings at this point. I know I will loose everything but it is worth just getting this thing off.


Operating System
Windows Vista Home Premium