XML Membership Provider available?

Question

I'm wanting to implement an authentication model based on an XML file for storing the username, password, role, profile, etc. Is there any native XML authentication provider or one out there that I can use? I plan on using a format like this:

<?xml version="1.0"?>
<admin>
  <john>
      <name></name>
      <password>A15F69189FDD3C0A64F82C0AD73C319020351978</password>
      <email></email>
      <phone></phone>
      <etc></etc>
  </john>
  <mary>
      <name></name>
      <password>WERG4589FDD3C0A64F82C0AD73C319020351978</password>
      <email></email>
      <phone></phone>
      <etc></etc>
  </mary>
</admin>

<users>
  <alex>
      <name></name>
      <password>SDFHW345C0A64F82C0AD73C319020351978</password>
      <email></email>
      <phone></phone>
      <etc></etc>
  </alex>
  <debbie>
      <name></name>
      <password>A15F691STGW4560AD73C319020351978</password>
      <email></email>
      <phone></phone>
      <etc></etc>
  </debbie>
</users>

The reason why I chose XML for the "database" is because I want to keep my app extremely portable. A database is just too hefty. I plan on using a data access layer so when the user base gets too big (like what 2500 users?) then the migration to an SQL database provider would be seemless. Can any gurus help point me in the right direction? If there isn't any XML provider out there, so far i will have to write these methods:

ChangePassword
ChangePasswordQuestionAndAnswer
CreateUser
DeleteUser
FindUserByEmail
FindUsersByName
GetAllUsers
GetNumberofUsersOnline
GetPassword
GetUser
GetUserNameByEmail
ResetPassword
UnlockUser
UpdateUser
ValidateUser

Any feedback or advice on this would be greatly appreciated. It seems a little intimidating at first glance.