If you want to only have one antispyware app installed you should look into either AVG Anti-Spyware or Superantispyware. They are both free.
http://free.grisoft.com/doc/20/lng/us/tpl/v5

http://www.superantispyware.com/

Maybe also add Spywareblaster for extra coverage.

http://www.download.com/SpywareBlast...-10486084.html

Normally I shy away from giving personal opinions, but if I had to choose btwn. Ad Aware & Spybot, I would lean towards Spybot because of the Tea-Timer Protection.