simple web service problem

Question

Hi, I'm trying to build a simple web service that authenticates users. I've seen a couple of examples on C# corner the only difference being that the data is held in a back end database. I want to keep the data as simple as possible and use an xml file to store all the data. The problem I have at the moment is that when I try to read the xml file access is denied. I've played about with web servers and web sites a little to know that trying to read and write files to a web server can be pretty tricky. Can anyone suggest how I can extract user information from an xml file on the web server and write data back? Thanks