I use Windows Defender, Ad-aware 2007 and Spybot S&D.
I have been using the new version of Spybot for about a month now and notice it now runs 24/7 which uses a fair amount of resources.
TeaTimer.exe which is part of Spybot is currently using 36MB memory usage, which if far more than my Antivirus and Firewall put together...
I don't know if it is possible to disable this TeaTimer.exe or not.