My father once had more computer virus's and spyware etc on his computer (amazing it could run) then I have ever seen. Even though he had real time protection of both anti-virus and anti-spyware he never actually ran the scans!

The thing about free anti-spyware is that most of it you have to pay for real time protection and lilkid mentioned that it bogs down your computer. Most of them come with a free trial of real time protection. Which just basically means we'll try to intercept spyware that tries to get onto your computer before it gets there. For that the free versions I suggest are Super AntiSpyware and SpywareBlaster www.javacoolsoftware.com can both be found there. I think the updated version of Spybot Search and Destroy has some real time protection as well. They have worked out very well for me just make sure its all updated and enabled your also going to need some spyware scanners its good to have a few as long as they don't conflict (they normally tell you I haven't had an issue even though Lavasoft and Spybot say they might remove stuff accidnently from the others program.)

What I use
Spywareguard -Spywareblaster ( http://www.javacoolsoftware.com )
Spybot Search and Destroy http://www.safer-networking.org/index2.html
AVG AntiSpywarehttp://free.avg.com/ww.download-avg-anti-spyware-and-anti-rootkit
SuperAntiSpyware http://www.superantispyware.com/

I have lavasoft but honestly I don't use it that much because the new version sucks. Don't forget anti-virus and firewall and you'll be set if you keep everything updated and run a scan every so often. I also use ccleaner to clean out junk and WinPatrol to keep me alerted on stuff/further protect and configure my system. http://www.winpatrol.com/