I ran a beta version of avast 5 on my old P3 laptop. I uninstalled via add/remove and ran the uninstaller too, but not in safe mode. I also, carefully, removed any trace in the registry.
I installed the final version of avast 5 and everything is fine. I'm running only the file and web shields, have streaming unchecked, and have most of the persistent caches off.
I also did install avast 5 on my desktop after removing avira, and everything is fine.
Both machines are XP Home SP3.