Do you have permission to edit the file? I would assume, but just try this:
Go to the CSWeapon.xml and right click. Click "Properties" then click the "Security" tab in the window which will appear. Find the button that says "Edit" and it will open a new window. In the top list box, click your user (it should say something like Users- David PC). In the bottom list box, click the top check box (Full Control), then click OK. Hopefully that made sense.  ;)