OllyDbg <-- not a bad dissassembler . I think if u wanna look into an exe, you wanna use olly. This wont let you see the original source code file that produced the exe, but it will let u see the assembly code which is enough for making some hacks.
There is a really good thread here for getting started with OllyDbg called "OllyDBG & CO" by tanelipe. It will show u how to make your own multiclient, enable pm commands and such. That would be an ok starting place. Ah... also search for an Assembly Language Tutorial, get the hang of that too. (Otherwise its gonna look very confusing, like some alien language)
When ya get used to seeing that (and assuming u know some programming already), start searching some sample code google for hooks and DLL injections.