No such thing. Unless you are smart and conservative with how you use your computer, you are going to break it.

All you can really ask from an antivirus program is for it not to mess up your computer and not slow it down. I always tell people that I would rather have a virus than have norton installed.

Eset's nod32 is one of the better antivirus program, and its "smart security" suite will not make your computer run any slower or take up that much memory when running. It even has a 30-day trial.

Free Smart Security Downloads from ESET