I've been a long time user of Avast not because of its real time anti-virus protection but because I like its features like browser cleanup and remote assistance.
However recently my system was having more and more bursts of unexplained activity in the background even when scheduled AV scans were not due. In sysinternals Process Monitor you can see that Avast is always chugging away at something.
Today I decided I'd had enough and uninstalled Avast. What a relief! The computer is much quieter and process monitor shows minimal background activity. On top of that programs start up quicker, websites that used to takes ages to load because Avast was scanning them now load much quicker. Flash games start up faster and use less CPU time and twitch TV runs perfectly in the browser at source quality without the choppiness.
Currently running without any anti-virus protection until I decide on an alternative. But anyway no AV has ever stopped a malware download on my machine that I wouldn't have prevented myself.