I use Avast with it running in the background. I have the Pagefile set on a different drive and is system managed. (I will soon be adding a 'boost' stick.) 4 gig of Ram and 2 gig of video Ram. a Dual Core 2.66 intel CPU.
The fault is just random enough to be annoying. Having opera start where I left off does help but sometimes your concentration can be broken and messages can be lost before posting.
Seems to be behaving since I turned my system into a dual monitor system. Weird
running in background is ok. i don't recall avast doing a scheduled scan when i used it, but check you settings just in case. and avast has a hardened mode that can cause problems when turned on (by blocking certain processes/programs). i think it is off by default, but peek inside the settings to make sure. avast sometimes has a update checker that might run on a schedule and a net work scan (smart scan?) ... i'd make sure they're not scheduled to auto scan also.
avast sometimes has a large emergency update that runs, seems to be separate from their regular update service. check your avast logs to see if that update occurs when you have problems (unlikely, but worth a look).