As has been previously mentioned, once you think your PC has been compromised, it is difficult to be 100% sure that you found and removed everything. Even if you cleaned every file, it is still possible that a vulnerability still exists on your system that would allow it to become compromised again.
That being said, there are useful online tools to help determine if your PC is compromised. I would start with the free tools offered by Trend Micro:
HijackThis is somewhat technical, but will examine your hosts file, look at your Browser Helper Objects, and inspect other common vectors that cause your browser to display unwanted content.
Another option is to use a rescue CD to scan your system for viruses. I have personally used the following one from F-Secure and found good success:
If you use the F-Secure recovery CD, you will need to download the .zip file, extract the .iso image file and burn it to a CD. Then boot your PC from the CD and follow the instructions. The anti-virus program will attempt to download the latest virus definition files from the Internet before scanning your PC.