Microsoft Security Essentials is great for your everyday protection; much more efficient (read your computer runs faster) than AVG or Avast
If you're concerned that you already have a corrupt file or virus/worm/trojan/adware/malware then
Malwarebytes is a fantastic scanner; it includes a boot sector scan IIRC.
Downloading and running
CCleaner will remove any files your computer doesn't need, like registry files left over from uninstalled programs. After my scan, I had 200+ files. It'll also fix (or at least identify) any missing/damaged program elements.