ThreatFire is a NEW Behaviour Based Anti-Malware From PC Tools, which offers definatin-less, realt-time protection. It claims to offer comprehensive protection against viruses, worms, trojans, spyware, rootkits, keyloggers & buffer overflows. Its a 14KB beta download. The shift these days is towards behavior-based protection...

When minimized and working, my Vista's Task Manager showed 1.3MB and its TFService showed 2.9KB memory usage.
http://www.threatfire.com/download/