Spybot and addaware work well together for me and I have an old,slow computer.Each catches a few the other will not,at times.
I liked Spybot best at first but now that I've gotten used to Addaware too,I feel about 50/50.
That's the free versions of Addaware and Spybot.IIRC the Spybot was very easy to install but Addaware took a little more effort for me(truly computer illiterate before).