If it actually is System Doctor you have, it's a rogue, and installation is via a Smitfraud variant.
Under no circumstances should anyone buy the darned thing.
Effective looking removal instructions
here. To the original question, my opinion is that Spybot is still worth it- there are some better freebies, and maybe better protection strategies- but Spybot is still actively supported, has a good forum, frequent updates, and some useful tools. The detection and cleaning engine is, I've read, a bit past it for many of today's malware, but I've never had to put this to the test.
AdAware...well, um, it's got a pretty interface...
Good free scanners/removers: definitely
Superantispyware, also consider
A2,
Cureit (by DrWeb, standalone download-to-update virus/malware scanner) amongst others.
Good prevention strategies include SpywareBlaster by Javacool, MVPS Hostsfile (I use Hostsman to manage/update this), maybe Comodo Boclean antitrojan, Threatfire behaviour blocker, and along those lines, you might want to consider whitelist based applications, such as HIPS, and/or virtualisation like Sandboxie.
Layer.
I use Avast Home, Threatfire, Comodo Firewall 2.4, CMF, Hosts file, SpywareBlaster, Firefox with Noscript and AdblockPlus, and clean all temp files after every browsing session with CCleaner. Seems to work. All free.
Also highly recommend
Secunia software inspector. Alerts to a very large range of software typically installed on a PC that may need patching.
Hope that helps.