Reverse Enginnering a exe file ……….

You can try a diassembler, but that is only good for converting it back to assembly code.

Also, there are technical reasons as to why you won't get the full source code.

You can try IDA Pro, and there is a Hex - Rays plugin which converts an EXE into C code, but don't expect too much. However, it expensive and very complex to use.

Start with OllyDbg. It works good enough for most purposes. http://www.ollydbg.de/

Also, out of experience I am telling you don't expect to get the full source code back. If you need it for debugging, the assembly code should work fine.