Heuristic Malware Security: Cmpatable?; ADVISE on specificapplications

I have a Dell Dimension 3000 running XP with SP 2. To secure it, i
have installed the PC Tools Internet suite, and also the Windows
Defender antispy. Both PCT Internet Suite (its component named
Threat Fire) and Windows Defender are heuristic. QUESTION: (1) are
all heuristic security applications compatable? ; (2) I am considering
also adding Dr. Web Cure It--is it compatable?; (4) I want some very
light weight and intuitive heuristic apps--can you ADVISE on light
weight and intuitive heuristic apps? They must be compatable.
Thanks a lot!
Best!
Robb