SpyBot - Search & Destroy
Resident add-on
Freeware, ⌐'2000-2003 Patrick Michael Kolla

Spybot-Search&Destroy is an on-demand spyware and malware scanner. That means it scans for threats whenever you choose to do so. It also offers some preventive measures like it's Immunize function.

Resident is Spybot-S&D's permanently running partner that does on-access scanning. This means that it will instantly detect and counteract to threats and unsolicitited system settings changes.