anti-virus alternative

Ok so i got this new older laptop. It only has 256mb of ram. I basically use it for everything except games. I take it to school and do homework on it etc. But i install AVG anti-virus/firewall and it uses up like 50 – 100mb of memory. Instead of upgrading my ram, does anyone know of an alternative application or solution that doesnt suck up my precious ram? My computer goes so incredibly slow with it on. I do have ad-aware ad-watch though. So im atleast protected against most adware/spyware etc.