flash drive resident antimalware app?

A while back someone suggested having, as part of your antimalware arsenal, some app that could be installed on a flash drive which could supposedly detect any spyware / malware on whatever computer the flash drive was plugged into. (I think the app was DOS-based, and it could work even if the system was severely compromised, although I'm not sure about those two points.) Could anybody tell me about this app? And would it detect a key logger?