I have seen this topic posted here and there a few months back, but I've not seen any true resolution anywhere. Like many others, I experience gradual performance degradation of my Vista system when running Avast 5.0.x (currently x=545), in particular, when using the Behavior Shield. A reboot clears things up for a while, hours or even a few days, but eventually things begin to slow back down. It is most noticeable when browsing the Web (IE or Firefox) -- if I disable that particular shield, performance improves drastically and instantly.
I've tried many reboots, deleting Avast and reinstalling fresh, etc. Ever since I've upgraded to version 5.0, this problem always eventually comes back. I'd prefer to have the protection of the Behavior Shield, but for now, I keep it disabled.