I have the same problem.
Since the last avast update, windows 7 doesn't recognize my scanner (it works if I plug it on my other computer). I am not sure avast! was the cause of the problem but i tried to uninstall avast! (with avastclear.exe) and install it again. Then : "The installer is unable to initialize early avast! self defense with error 0x00000005! Aborting!".
Then I tried to clean the registry with Ccleaner (which did find some issues related to avast) before reinstalling but it didn't solve the problem.