If the program is open source, you can Google for its source, it isn't you can't
The most you can do is disassembling it, the results of a decompiler are unreadable by humans
There is no way to get the exact code from an exe, but there are some decompilers available that give you some code. I did a quick google search for one and got this one, http://www.backerstreet.com/rec/rec.htm
I don't know how it is, I've never used it. Google it and see if you can get any others. Remember, you are not going to get the exact code.