When going to delete the present admin account you first create a brand new one to replace it before removing the original. Without logging into the new admin account you'll need to perform a repair install of Windows in order to see a new admin created in order to remove the first.
You will run into the access denied problems of not having the correct permissions as well as administrative access to start with simply trying to remove any admin related files and folders. The only thing to try now would be the repair install option for XP or trying to first create a new admin account and then using a free reg mod tool like Takeownership. http://www.howtogeek.com/howto/windo...menu-in-vista/