The following are all free:

Malwarebytes Anti-Malware:
http://www.download.com/Malwarebytes-Ant...
Download, install, update and select Full Scan.
Remove all infections that it finds after scan.

SUPERAntiSpyware Free Edition:
http://www.download.com/SUPERAntiSpyware...
Download, install, check for updates, then select Scan your Computer, select your drive and select Perform Complete Scan

Spybot S&D:
http://www.safer-networking.org/en/downl...
Download, install, update and Immunize, turn off Tea Timer then click 'Check for problems' then when complete select all and then 'Fix Checked'
Open Spybot, click on Mode, select Advanced, click on Tools then Resident and untick Resident 'Tea Timer'
Do NOT untick Resident 'SD Helper' as that is your protection.

Spyware Doctor Free Basic Edition - Realtime monitoring:
http://pack.google.com/intl/en/pack_inst...
Its free with Google pack, untick all boxes except Spyware Doctor. Download install, update and run.

A-Squared Free:
http://www.download.com/A-squared-Free/3...
This is not a limited version but a full tool to clean your computer from Malware, Spyware, Trojans, Backdoors, Worms, Dialers and Keyloggers.
How to use:
Download, install and it will ask to check for updates, which should be allowed and will take a few minutes initially, it will then restart the program for updates to take effect. Click on Scan and ensure deep Scan is selected then click the Scan button.