A reference or guide to write a application with database XML

Question

Hello all, I used write a application with database use MySQL, but now I had a required that write a Application with database is XML, so I don't understand and strange with it (XML) (write in Windows Form C#) so I need a reference or guide, who have a link to reference or example, simple about it please send me, thank very much.
 
Below is my required of application.
Write a application with database use XML, have 1 table with a field (userID, username, login name, password) write a program with demand
1. Check or test 1 login valid
2. Change a password (note: password is only save value hash of password)
3. Add or remove 1 user.