While I feel it's probably not necessary as some of the best code in the world is open source and have several other reservations about it, I'll still do the nice thing and answer your question. You could register your work with the U.S. Copyright Office if you wanted to be EXTREME about it http://www.ehow.com/how_5956351_change-copyright-exe-file.html
it depend on your choice. you can do it by your own country copyrighted law (only for your own country) :)
Copyrights are a Courtesy and Convenience if you do not legally Copyright with the Patent and Copyright Office.
You can use Prior Work and First Use in some cases. But, it is getting harder and harder and more and more expensive to get a genuine legally enforceable Copyright.
Then it can get extremely expensive to prosecute someone who has stolen your code or idea.
For an xlt, you can password protect it...
For an .exe you have to protect your code...
One method that used to work in the old days was to create your source and executable disk set and mail them to yourself in Certified/Registered Mail. do not open them when you get them back.
What this does is put a legal US Government Time Mark on your package proving that you wrote the code before someone else "stole" it.
But, unless the idea is *really* unique and *really* marketable, it is generally not worth the ffort.
Except in this one case.
If it's something you really like playing with that *might* be stolen by the "big boys" and you want to right to continue to play with it and sitribute it without paying the "big boys"... You may want to prove that you had the idea before they made theirs public.