Download and run the free version of Revo Uninstaller.
As I recall, the free version or Revo does not handle 64 bit applications fully, where as the Pro (buy) version does.
Not sure whether that is relative in this situation though.
Avast is not a 64-bit application.
Revo free won't even show it in the programs list if it is.
It is showing in the list on a Windows 7 SP1 Home Premium 64 bit system.
Revo free should work fine.
Thanks.