Spybot S&D was a good tool in it's day. You can easily run ST with others especially Ad-Aware 2007 being run with ST on both XP and Vista alike here. One likes to catch what the other misses!
The ST freeware will generally find more data miners then Ad-Aware right off. Yet Ad-Aware goes after browser hijackers while ST puts up an active protection similar to a firewall. It will even prompt at times asking if you to allow or deny when going to remove a program.
The best advice always however is still being familiar with several and never counting on any one program alone. That goes for antivirus programs as well as adware and spyware removers. That's one reason why you see a large number of retail programs for various things like IE protection, malware removal, adaware/spyware removers.
One other thing to note here is Grisoft's antirootkit utility found on the same page as the free version of AVG. Comodo's BOClean is another free tool to have onhand as well.
http://www.nsclean.com/trolist.html