After using Ad-Aware for a while, I have uninstalled it. While on the whole it is very light, at times CPU use does get high which is why I ditched it. Also I didn't like the fact that new releases have to be manually downloaded and installed. I guess this is okay for major releases, but for minor updates I would prefer if Ad-Aware could do the updating on its own. For now I have switched to Webroot SecureAnwhere. It's not free, but you can get a 6 months trial version here if you have a facebook account. I haven't had much time to use it you yet, but it does seem to live up to its claim of being very light.