AVG is pretty popular, but the later versions are pigs and really slow your system down if you're using Win Vista or Win 7. I also had frequent crashes and system lockups using AVG with my version of Win7 64 bit. After months of research, I figured out it was AVG causing it.
Switched to Avast free and the problems disappeared. A combination of Avast (free) and Spybot S&D (also free) will give you plenty of protection at no cost.