Just a thought: maybe those frequent users reporting no problems here in this topic have been updating Avast from previous versions for a long time, and the problem arises when installing anew the latest Avast version (or one of the versions released during the last few months)?
Or this might be happening with "new" Windows XP installations, where "Validation" / "Registration" and all critical Windows updates are not yet applied?
I don't have any statistics about installations failing with older Avast Setup releases in comparison to Avast 6.x Setup, but it "seems" that several (unrelated?) installations are somehow failing (BSOD, crashing and/or words like those).
Again, just a thought of a possible current situation, with no real objective data to back it up.