Avast is a pretty good home solution. The Active Protection can slow down some computers, but overall it's a pretty good product. One of the main reasons I like it is because of it's boot-time scans which is a very good feature that other free anti-virus solutions don't offer.
Just stay away from norton & mcaffee altogether. They will definitely slow your computer down and only catch a fraction of stuff coming in.
AVG free is a good solution as well, if you're interested in just freeware.