I've run those 3 daily and have kept my Windows 2000 system clean.
I'm now in market for a new PC and the move to the world of XP and the company I'm dealing with prefers to install TrendMicro (Free for 1 year). Claims Norton does not update their virus protection frequently enough. Regarding spybot and ad-aware they claimed it would slow down my PC but these are not resident in my system tray so I dont think that is the case (unlike SpyBlaster).
Should I stick with what I'm comfortable with or is TrendMicro a better product and a replacement for all 3?