I am building an instant messenger and I want to copyright it.

The first versions will be available ONLY for Windows

However in a year or so I will optimize the code to work under Linux.

I want it to be free software, so, do you think that I should use the GNU General Public Licence, or just copyright it and make it freeware?

Making it GPL will take away your control of the project. Do you want someone to fork your code and created a brand new version where you dont have a say in the developement process? Honnestly though if you dont care about who uses the code i would suggest you BSDL it.

Could you please tell me more?

Releasing it under GPL means anyone can get the code and create his/her own version (but can't remove your copyright) released under GPL as well. If you release it under BSD anyone can get the code and release a new version (without removing your copyright) under any license (may make it commercial).

So IMHO when you release something under GPL you don't give away your control. You force the code (and derivatives) to be GPL forever.

just ask this question: do you ever want your code to become commercial code, or included in a commercial product?

theres also other licenses you can look at

I would favor GPL - if anybody ad new code to the program (in another branch) you can use it. Plus you can take pieces from other programs and add it to you program.

Relasing the code under a GPL / BSD licence means people can fork it, but only if there is a legitimate reason. an eg of this is the x.org fork of XFree, which was started because the x.org people wanted to play with some new features and the XFree people had a pace of development too slow for the x.org people. (I think I am gettting this right, I only read about it after the XFree licence change)

An example of this not happening is Linux itself - after 13 long years Linus is still the keeper of the kernel.

If you have any other queries, perhaps you should look at this :
http://www.fsf.org/licenses/gpl-faq.html

Anyway, the main reason I wanted to post this is a suggestion :

When you write your messenger, write it using the wxwidgets libraries (http://wxwindows.sourceforge.net/) It will allow you to port with a simple re-compilation, rather than having to re-write your GUI with KDE / GTK / whatever. IIRC, it even has a portable socket library, allowing all your networking code to be platform independant as well.

Otherwise, you have to re-write all your socket code & GUI code for each supported platfrom, which is a pain.

Other than that, have fun