The following tools and utilities are free of charge and work very well.

Anti-virus Programs (use only one per computer):
Avira AntiVir Personnal
Avast Home Edition
AVG Anti-Virus Free Edition

Other Programs:
Malwarebytes' Anti-Malware
Steven Gould's Clean Up!
Spyware Blaster
Auslogics' Disk Defrag
Opera Web Browser
Mozilla's Firefox Web Browser