Resolution:
Whenever a subject callback is created, if the subject is new (no other callbacks exist for that subject) then two mutexes and a conditional variable is created (this consumes three handles) to protect the callback in a multithreaded environment. So, with 2000 subject callbacks each with a unique subject (2000 * 3) 6000 handles are used.In Windows 2000 you can have 65K handles open per process. These three handles per unique message type are used even in single-threaded apps. No such penalty is incurred using process callbacks.