It's called reverse engineering. A number of people here (including myself) will continuously trace through the assembly code and comment/document what it does. This wouldn't be necessary, should we have the original C source code as we could simply modify that instead and recompile it. But since we don't, we have to do it the hard way - trace through assembly, modify the code and redirect some calls/jumps to a DLL code. You sould probably learn how to use a debugger. A good free debugger and a favorite of many is Ollydbg, a more professional one would be IDA Pro, but it's not free.