Hi,
Sometimes computer freeze due to bad .DLL files not loading up or because of incompatible device driver. If your computer keeps freezing then at some point you may assume it's a virus. If you have the latest virus definition for your antivirus promgram then run a full scan on your system.
I update my virus definition everyday. It's very critical especially if you have broadband. Make updates everyday and scan everyday. Trust me you can never be too carefull. If you happen to find a virus, then just remember that you need to undo what the virus has done to your computer, by means of editing the registry. Even if you do get rid of the virus, it will leave traces on your registry, so when you boot backup your computer will run the virus again. So you're back to square one. Find out what virus you have then find out how to undo the effects of the virus, by going to the website of your product. They should have some knowlege base document or something.
In some instances the virus will can affect your antivirus program from not loading up and running. In this case you can get an external comptuer to scan your hard disk.