Need help learning how to hook

I'm having a hard time learning how to hook API functions. Basically, I want to take a running process, and have it execute MY API function instead of whatever it calls. What I need is for someone to show me the simplest way to accomplish this with WELL DOCUMENTED source code. This person also needs to be able to answer any (reasonable) questions I have regarding this topic. The source code needs to be in C/C++ (using MFC is OK). *Added on 10/07/03 While I want to learn how to hook in general, there is a specific task I am trying to accomplish. There is a program that I run that copies text to the clipboard. I want to hook OpenClipboard() so that I know when then program writes to the clipboard.* ## Deliverables 1) Complete and fully-functional working program(s) in executable form as well as complete source code of all work done. 2) Exclusive and complete copyrights to all work purchased. (No GPL, 3rd party components, etc. unless all copyright ramifications are explained AND AGREED TO by the buyer on the site). ## Platform all 32 bit windows versions