I just upgraded to the latest version of Avast and I've had severe slowdown problems.
I ran Ad Aware. Checked task manager and AshMaiSv.exe was taking up 80-90% of the processor. When I terminated the process, Ad Aware ran much faster.
Grabit would only half run. It would clear out old news articles, then not download any updated headers. I stopped the Internet Mail provider. Grabit started working immediately.
I've only just rebuilt this computer, so there is currently very little software on it. The problems I describe only happened when I upgraded to 4.6.603.
My PC is Windows XP SP2 + latest updates. AMD XP 64 3200+. 1GB ram. 250GB sata disk. Asus A8V-E Deluxe PCI-Express motherboard. Ge-Force 6600GT graphics. Latest versions of Firefox, Thunderbird, Grabit, Avast, ZoneAlarm, Ad Aware, Spybot S&D + TeaTimer.