I think I have a virus!

Hello, Gary R has helped me here before, and I believe I need help again. I went online today and noticed I couldn't access facebook. I looked for a solution and I used FixIt by microsoft, which corrected a problem called "execution" something, don't remember. FixIt corrected the problem. Then I got a windows error saying that windows shut down unexpectedly and also that ATI was not working properly. I don't know what to do. Please help me.