How to find given content of node and modify xml file using C++/CLI

LoginsAndPasswords.xml looks like this:

<?xml version="1.0" encoding="Windows-1250"?>
<AllUsers>
  <User>
    <id>2</id>
    <login>a</login>
    <password>a</password>
  </User>
  <User>
    <id>5</id>
    <login>b</login>
    <password>b</password>
  </User>
  <User>
    <id>7</id>
    <login>c</login>
    <password>c</password>
  </User>
</AllUsers>

At first I insert some login name into loginBox. Then before i add new user I want to check if that login already exists in my xml file. For example I would like to check if login named loginBox->Text=b already exists in xml file. If yes I want to show MessageBox("Given login already exists choose another"). If no I want to create new User in my xml file with given unique login(loginBox->Text), given password(passwordBox->Text) and id greater than max id value of all users.