I've updated to 827, everything now seems fine.
After getting the access denied error, but before discovering this thread, I found the avast folder had no permissions for any user or group. I tried adding administrator with full, which let me start the gui, but somehow the permissions reset (to blank). I then gave full permission to administrator, everyone, system and service, forced permissions to inherit to all child objects and was then able to run the gui again *and* update.
Reboot requested and done. avast is now working =]
ps: os is w2k and avast is installed in a folder other than "c:\program files", in fact, it's installed to "d:\avast"