Free antivirus, spyware, adware, Malicious Software scanners and removal tools:

For Windows machines:

 - Microsoft Security Essentials - An active scanner for constant protection - http://www.microsoft.com/en-us/security_essentials/default.aspx

 - AVG Free Antivirus - Another active scanner - http://free.avg.com/us-en/homepage

 - Avast Free Antivirus - Another active scanner - http://www.avast.com/en-us/index

 - Emsisoft Emergency Kit - A scan-only program - Fantastic for removing infections - http://www.emsisoft.com/en/

 - Malwarebytes - Another scan-only program - http://www.malwarebytes.org/