Go to the Joomla main folder and right click, select properties and un-tick the read only which usually has 3 positions ticked, un-ticked and a green square which is the default.
After removing the tick on the main Joomla folder click Apply
then select to Apply changes to this folder, sub folders and files
then click OK
Now when you right click on the Joomla folder again and select properties, it should show the box filled in with the green square.
Now try and write to your script again.
BTW how are you trying to write to the script?
Also make sure you are logged in as administrator not a just as a user.
If you are logged as an administrator when you right click on the Joomla folder and select properties, select the Security
tab and make sure that the allow column is grayed out and all the boxes are ticked - as admin you should have full permissions for your XP.