Decompile Program

So, I've been getting into hacking, and making hacks and stuff. And I wanted to look at a source, and learn from it. It's an old ass source, and its outdated and detected. So I'm not trying to crack it or anything. But I don't know how to decompile it and stuff, to get the source. I've tried a few methods, to no avail. I have a .dll file and an .exe of it. If anyone could help me, it would be greatly appreciated.

did you try jetBrains dotPeak yet?