As about the background scanning, well it's really up to you if you want to leave it enabled or disable it. In any case, don't change the registry keys manually; instead, use the avast "Exchange 2000/2003" provider GUI to change this (I think it's the first checkbox in the provider's configuration).
Background scanning basically means that after each and every virus database update, the whole Exchange store will be re-scanned using the new definitions (on the background). Of course, this IS a performance hit, but will keep the store fit.
If you uncheck the option, objects will only be scanned on-access. I.e.
As about the thread count, I'd recommend leaving it as it is (i.e. use the default value).
Thanks
Vlk