I'm coming pretty late to this thread, but for the future I highly recommend installing Microsoft Security Essentials. It's easily the best free antivirus software.
Oh, and like Yogi said: Starting your computer in safe mode is always a good first step. You can then scan your drive for any malware, etc. while it's inactive.
If you're new to this whole thing, think of it this way: You should maintain your computer like a
property maintenance expert would maintain your home. If you let things go for too long (in this case, virus scans), then it will be significantly more difficult to restore you computer. This also applies to maintaining your hardware, like cleaning your case every few weeks.