How can I change a Windows .exe to something that is compatible?
 

The Title says it all. I have followed the instructions from this site http://www.myabandonware.com/howto/, but then I get this notification that the program file is a Windows executable, and one that does not work on DOSbox. How do I get it to one that is compatible?

please help!

You don't.
The only way to run Windows programs in DOSBox is to run Windows itself inside DOSBox and then the programs in there.

Oh, relating, but totally different question, 5th.

Is it possible to transform Win16 format to Win32, without decompiling? I have a very little knowledge about difference between it, so I have no idea, if it even theoretically possible... :)

Nope

No. Completely different APIs.

Tsk. Pity. I had slight hope that I can start all this Win16 games in my brand new Win8.1. Will not happen. Ok, thank you!