This program comes highly recommended from Consultsteimel.  I use the free version if you think you need the pro version by all means get it.  The free version works great, download it, update, run it and it gets rid of anything nasty.  The full version comes with real time protection.  I have a full time antivirus that takes care of that.  While most antivirus programs don't deal with malware all that well, i don't want to have more then 1 program scanning my active documents.

Malwarebytes' Anti-Malware   

Panda cloud anti virus is the first anti-virus program to use the "cloud" concept.  The cloud refers to the internet and allows programs to use remote locations to do there processing.  This allows the program that runs on you computer to be very small and lite.  This is a huge difference compared to programs like norton 360 and Mcaffee that take up huge amounts of resources.  These programs do provide an enormous amount of tools but for the average user a good anti-virus programs and some smart web serfing is all you need.  The interface is super simple and pretty much is just run it and forget it.  You can also right click on any file or folder to have it check it on demand.  I have been recommending and using Panda cloud since it came to beta.  I haven't had any problem running the beta version except for the fact that it didn't run on Windows 7.  Now that Panda is officially out of beta I still recommend using it.  Its free which is awesome.  The release version focuses even more on the fact that it can be lite so don't worry about it slowing down your computer or running it on a old machine.  Its also good for windows 7 now. Gawker Media : Source

UPDATE: I'm sad to say Spy bot is not longer a recommended piece of software.  Bloat and lack of detection has added to this programs demise.  Please check out Malware Bytes for your free spyware problems. 

I have been using this software for years and continue to recommend it to everyone. Its very easy to use and its free. This software has not been bloated and keeps getting better with age.  Spybot compares your files to a large repository of spy ware and ad ware.  It also had a good immunization database which protected you from sites that are hazardous to you.  This latest version provides an even smoother updater so that you run it with the latest list of spy ware. 

Check it out here:

UPDATE:  Adaware is no longer recommended by CS.  Bloat and lack of decttion has added to this programs demise.  Try malware bytes for your free spyware dectection needs.

Lavasoft's Ad-aware program has been in the malware game since the beginning.  The basic still remains to be free and while it does not scan your background files it does do a fine job when you remember to run it.  The update does run in the background and continue to keep you up to date with the latest malware detections. 

I have been using this software for years and continue to recommend it to everyone. Its very easy to use and its free. The ones you pay for provide more advanced functions but you don't really need them. Check it out here: