It would probably be better to wait for someone that has used both Prevx and Spyware Doctor because I have never used either but my situation is similiar. I DO use SpywareBlaster and SpywareGuard plus I have the Realtime protection turned on with MSAS. I also use Spybot's host's file. I just tried a 14 day trial version of Ewido which is an excellent trojan scanner and also offers a Process Guard similiar to the others. I noticed immediately that it took longer for any of my regular programs to come with when I clicked on them but I stayed with it for the 2 weeks. Yesterday the trial was over and the Process Guard was disabled and everything works normal again. What I am saying is that it might be a trial and error procedure for you as it was for me, even though we are just trying different programs.
I have discovered that I cannot run both Prevx and Spyware Doctor's resident protection. I already use Spyware Guard and Spyware Blaster which, I believe, covers 90% or more of the same protections as Spyware Doctor's resident protection program.
If I do run both, my computer consistently hits 100% processes and it becomes barely useful. If I turn off either program things run fine (since I run so much anti virus/spyware stuff, my pc's processor has always run a bit hot as it is).
Any suggestions which one I need more given the above scenario?