There is a online scanning tool that most dont know about from Microsoft. It will scan for viruses, clean up the registry, tell you if you need to defrag and any open ports you have. I recommend using this to scan if you have a PC. There is two versions one for XP systems and then the latest windows vista or windows 7.... This should be done at least once a month to give your system a tune-up!
Visit Microsoft Windows Live Onecare Saftey Scanner!