How to modify a 16 bit exe file

[firebat311@...]

Greatings all members:
I want to protect a program of mine from illeagal copying, but for 
several reasons I want to do that without modifying the source code. 
I am planing to write the protection code by my self ( in assembly ). 
The problem is that I don't how to attach the protection code to the 
16 bit exectuble file without modifying its source code and recompile 
it. I need somthing that can stick the protection code to the 
begining of the 16 bit exe file in way that make it executed first 
and then it will evaluate a condition, if the condition satisfied it 
will dispatch control to the original program, else it will display 
an error message and exit.
I spent the last couple of weeks digging the internet for a 16-bit 
packer, or encryptor whith supplied source code to modify, but I 
couldn't find any. I didn't find either any of the ready protection 
solution that suit with this case.
So if any one have an idea I will be very gratefull for teaching me 
that trick. I will also appreciate any other altarnitve ideas.

special regards
firebat311@...