I think the reason is that you can’t edit files in “Program Files” without administrator rights.
I had that kind of problem several times and I usually solve it by running my text editor with admin rights.
For example, to edit file with notepad in Windows 7:
Find it in your start menu, click on it with right mouse button. You should be able to choose “run as administrator” from the context menu. Now edit the file and you should be able to save it.