Hmm.. Indeed this was reported by RejZoR in one of the beta builds but we could never simulate it here. The upgrade path from v4.8 to 5.1 was tested on a number of different systems and we couldn't find a single problem.
Now, it would be really useful to find out what's different about your systems.
To help us analyze the problem, please ZIP the following directories
C:\Program Files\ALWIL Software\Avast5
C:\ProgramData\ALWIL Software\Avast5
and upload to
ftp://ftp.avast.com/incomingAlso, please add the files C:\Windows\System32\Drivers\asw*.sys and an export of the registry key HKLM\Software\ALWIL Software
We will analyze the files here and hopefully find out what makes your install different from a clean install.
Many thanks
Vlk