Technical,
it seems you like playing with fire
If you'd try to do a program update while using renamed setup.ovr to setup.exe, the setup process will do _something_ (I'm even not sure what). But it won't upgrade the product successfully - that's what I know for sure.
Believe me, I wrote that beast.
If you want the shortcut for the installer, use the one from Add/Remove:
rundll32 c:\PROGRA~1\ALWILS~1\Avast4\Setup\setiface.dll,RunSetup