All Anti-Spyware programs are only good for finding 25-40% (ish) of the garbage out there. Each has their strengths, but they can only find certain things (what is on their definitions lists).
Best bet is to have several so you cover all your bases. (3-4)
SpyBot is excellent, and it sounds like to me it has found all that it can, but that doesn't mean there isn't more.
Webroot SpySweeper, CounterSpy, HiJackThis, and AddAware would be good additions to have, and you would have most bases covered.
(unfort. not all, but that'll get ya' closer)
Update often.
I tend to use at least 4-5 diff. anti-spyware, and at least 2 if not 3 Anti-Virus programs when cleaning a customers computer.