I think it's more correct to say that the installation of Norton chooses to conflict with anything it doesn't like, Spybot being one of them.
Good list above, personally I find AdAware to be buggy and ineffective, would rather use the very well thought of
Superantispyware Others worthy of mention include Threatfire, SpywareTerminator (without Clam or WSG installed) and AVG AS, which will at some stage loose support, as it is about to be incorporated into the suite.