Thanks for responding.
You may not be aware of this but the .NET framework that Windows Xp supports is up to .NET4.0. It can't have anything above that.
So Visual Studio may 'support all' OSes but you need to code around the .NET4.0 framework to make it work for Windows XP. ANything above that and it won't work. So even your zip file won't work if it is based on the main program you created.