Separating delimited text into strings for an account list

I can't manage to figure out how to separate delimited text into strings. This is the format of the text "Admin:Pass". I need the first part into a string for checking the username and the second part into a string for check the password. I need it to read line by line and for each line check the username and password where they must both match for the login to be successful. The usernames and passwords are stored on an online text upload site for me to add new ones as people get the program. Here's what I have so far:

 Imports System.IO
Imports System.Management

Public Class LoginWindow

    Private Sub LoginWindow_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub btnQuit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnQuit.Click
        Me.Close()
    End Sub


    Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click
        Dim Request As System.Net.HttpWebRequest = System.Net.HttpWebRequest.Create(My.Settings.LoginInfo)
        Dim Response As System.Net.HttpWebResponse = Request.GetResponse()
        Dim SR As System.IO.StreamReader = New System.IO.StreamReader(Response.GetResponseStream)
        Dim LoginAdded As String = SR.ReadToEnd
        Dim ThisUser As String = txtUsername.Text()
        If LoginAdded.Contains(ThisUser) Then

        End If

        Dim ThisPass As String = txtPassword.Text()
    End Sub
End Class