SUPERantispyware – This is excellent for detection and removal, and regularly finds things that Ad-aware and Spybot both missed. Note that nothing is perfect, and I usually use two or even three of these scanners on a bad system. In terms of furture system performance, it is usually best to disable it starting with windows, but advise the user to run it manually every few weeks.
Spybot Search and Destroy – better than lavasoft’s ad-aware, and includes a really handy registry change monitor, telling you when something fiddles about in the registry. Excellent for spotting spyware activities.
ADW Cleaner This is an excellent one-shot program, which often can find and remove an infection, without installation or annoying trial messages.
Combofix – this is a collection of known fixes and is not very friendly. It will (eventually) reboot and throw up a report, which can give further information on infection. It’s good at fixing more serious problems.
Microsoft Security Essentials – If you’re running windows 7 (or Vista, you masochist) then this is good – it’s quite effective, quite lightweight, and doesn’t keep hassling about paid versions. It, however, doesn’t work on Windows 8 (the built in anti-virus is rubbish) and won’t work on Windows XP (too old.)
AVG Free Edition Excellent anti-virus, free for non-commercial use and regularly updated too. Still supports XP, though possibly not after April 2015.
As of the time of writing, all of these tools had free versions, or are just plain free.