I used to use Spyware Doctor and it used to be good.
Was v5 the last 'proper' free version? I remember that they pushed people to 'upgrade', but if you did, then you ended up with a 'free' version that detected spyware - but would only remove it if you chose to pay for a full version.
I ended up sticking with the last useful version (5?) for quite a while, but then they stopped providing updates.
Spyware Doctor used to be good - but IMO things like AVG (formerly Ewido)have come on a lot! Spyware Doctor is almost useless for me now. I've not looked in a while - can you now get a useful free version of Spyware Doctor again, or are they still trying to push people towards buying it?
/added - I've never had any compatibility issues with Spyware Doctor and ZoneAlarm for any of the versions that I've used.