It is possible to decompile some programs, depending on the complexity of the program and what language was used. Java and managed .NET programs can be decompiled to some extant due to the fact they get compiled in the first place to an intermediate bytecode, and also due to the fact that a lot of the original source is included in the final executable (e.g. class definitions, member names). Programs written in C/C++ can't really be decompiled, due to how compilation works for them. Expecially in the case of C, there is little information, if any, about the original source stored in the executable. Compiler optimisations also make it near-impossible to recover the original code.

You *can* dissassemble C/C++ programs, however this does not give you the source. What this gives you is the machine code, reformatted slightly for readability. It is possible to edit this code, however it is not the original source and so does not give you any of the information in the original source (this is what Unknownfile did with his patches). While you get portions of the program, you have no knowledge of the original context of the code, what the function was called, or how the function was intended to be called.