After running AVAST for a couple of days I can see how much AVG8 (and previous versions) were really slowing my system down, especially on start up, not turning on to loading XP, I mean on my desktop and the time it took for me to actually start using my PC. I'd start firefox and have to wait what seemed like ages until it loaded up, I'd say 10 to 20 seconds.
With AVAST as soon as my desktop is up I can start running programs. Plus it very customizable, I can update manually without stupid warning icons, choose whatever features I need, it’s much better than AVG (which I'd been using for years).
It also picked up viruses (not serious) hidden in old system restore files in the hidden system volume information folder on all 3 of my hard drives, AVG totally missed them
If you are running AVG, I'd say give AVAST a try and see if you notice a performance increase, well worth the change in my opinion.