AVG Free Edition is an excellent choice for a virus scanner also. It's free for non-commercial use and the updates never expire. Much easier on system resources than Norton and McAfee. Automatic updates, real-time scanning, etc...very nice.

http://free.grisoft.com

I also recommend Windows Defender as mentioned above. It has real-time scanning unlike most of the other free programs. Doesn't seem too bad on system resources either.

You can get it at http://download.microsoft.com