I think it was money well spent too, I bought mine several Christmases ago, same sort of deal around the end of the year.
I doubt the anti-spyware would conflict like many other resident anti-spyware applications don't conflict with avast. You could try it on boot and see what the Scanned count in the standard shield is, you never know it might have got a little less noisy at boot.
However, if you have gone for another Resident AS, e.g. SAS pro you wouldn't want another resident AS in the form of Outpost AS module also running as tey are more likely to conflict that with avast.