Microsoft Security Essentials
www.filehippo.com/download_security_essentials/

AVG Free Edition
www.filehippo.com/download_avg_antivirus/

SpyBot
www.filehippo.com/download_spybot_search_destroy/

AdAware
www.filehippo.com/download_ad-aware/

MalwareBytes
www.filehippo.com/download_malwarebytes_anti_malware/
(removal not prevention)