Comodo Internet Security is currently using 7.2 MB of RAM on my laptop. It's installed on all the computers at home with me. It sits there, does it's job. It isn;t amazing but it does the trick. It can be a little annoying sometimes when installing programs or updates but flicking it into installation mode shuts it up nicely. The interface isn't the best but it works. It covers all the basic security requirements of the average home user including anti-virus, anti-spyware and a pretty good firewall. My only real problem with it is that the automatic virus scan it does once a week can't be minimized. It has to be dragged to the bottom or side of the screen out of the way.

Oh, by the way did I mention that it was free?