I can verify what the OP states. I have uninstalled avast! on an XP x32, Vista x64 and W7 x64, and on all 3 machines, there were files / folders /reg keys left over. I uninstalled avast! via programs and features, rebooted, then rebooted into safe mode, ran the uninstall utility, and rebooted again.
I then searched for avast! and found 3 left over files / folders, then used Auslogics Reg Cleaner and it found about 5-7 avast! keys. I'm sure none of this left over junk is very important, but, it is there.