I'm a huge fan of Avast, but I have removed Avast from all my computers (windows based) and installed Microsoft Security Essentials to replace it.
MSE has tested pretty well in anti-virus tests, as good or better than Avast. It's lighter on system resources, and does not require manual registration every 16-18 months.
I would give MSE a big thumbs up and recommend you switch to it over any other free anti-virus. And I've never paid for anti-virus, and have never gotten infected.
The only time I got infected was with a paid version of Norton running my PC, installed by corporate, LOL.