SUPERAntiSpyware (If you disable it to start with Windows)
A-Squared free (This does have a process running, but it only uses very l ittle memory. However, you can disable the process from starting with Windows by changing the service to "manual" (go to Start > control panel > administrative tools > services > then look for a-squared and change the service to manual).
SUPERAntiSpyware looks kind of cheap and almost like one of those rouge antispyware programs, but infact it is extremely good and has excellent detection and removal rates. Their developer is always on the Wilderssecurity forum helping people and adding new definitions, fixing things, etc.