Better security for write permissions?

Hi, I'm running Win 2000 and have a site running php admin pages to write content to pages sitting on the web. I do have session authentication for the user to even get to the php admin pages. But here is the problem: My System administrator is very concerned about how many folders and pages on our web we have had to set an "internet/guest accout" with write permissions to those folders or pages for php to change the content. He said it is un-secure. We would like to know if there is a way to make php run as a certain user in IIS so that only that user has write permissions to the files? So that anyone browsing through the net can't write to the pages? What do most of you do? Angie