Let's see here, first of all, did you unistall AVG completely before installing Avast? The removal tool is listed here if you need it:
http://www.avg.com/download-tools
Next, are you running more than 1 of your anti-malware programs (excluding AV) in real-time? Such as Spybot Teatimer, SpyBlaster, and Ad-watch. It is generally recommended to only keep 1 of them running in the background, and use the others on-demand.
Lastly, do you have any other security products installed that runs in real-time?
Edit
Normal computer users who knows how to stay safe will only need Avast to be running in the background.