VB.NET Login using MD5

Option Strict On
Imports System.IO

Public Class Form1

    Private Function StringtoMD5(ByRef Content As String) As String
        Dim M5 As New System.Security.Cryptography.MD5CryptoServiceProvider
        Dim ByteString() As Byte = System.Text.Encoding.ASCII.GetBytes(Content)
        ByteString = M5.ComputeHash(ByteString)
        Dim FinalString As String = Nothing

        For Each bt As Byte In ByteString
            FinalString &= bt.ToString("x2")
        Next

        Return FinalString.ToUpper()
    End Function

    Private Function createUser(ByVal user As String, ByVal pass As String) As String ' Declares user and pass as strings
        'check to see if the MD5 has value of user exsists as a text file
        If File.Exists(StringtoMD5(user) & ".txt") = False Then
            Try
                ' write MD5 hash values of user and pass to a file names <user>.txt
                My.Computer.FileSystem.WriteAllText(StringtoMD5(user) & ".txt", StringtoMD5(user) & Chr(32) & StringtoMD5(pass), False)

                File.SetAttributes(StringtoMD5(user) & ".txt", FileAttributes.Hidden)

                ' If it works
                MsgBox("It worked", MsgBoxStyle.Information)
            Catch ex As Exception
                ' if it fails to write, then show an error message
                MsgBox("Something had went wrong" & ex.Message, MsgBoxStyle.Information, "Error")
            End Try
        Else
            ' username has already been created
            MsgBox("Username has already been taken")
        End If

        ' return values
        Return user
        Return pass
    End Function

    Private Sub Ok_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles OK.Click
        Try
            'check to see if the MD5 hashed value of user.txt and pass.txt are equal to the hash values inside
            'the file. We find the file by using IO.File.ReadAllText to find the MD5 hashed named file of <user>.txt
            'int real situations, you would replace the IO.File.ReadAllText with a connection to your database/server/website
            If StringtoMD5(UsernameTextBox.Text) & Chr(32) & StringtoMD5(PasswordTextBox.Text) = IO.File.ReadAllText(StringtoMD5(UsernameTextBox.Text) & ".txt") Then
                ' if is works, let the user login
                ' in this area, you can give a message to the database/server/website to allow the user to connect
                MsgBox("You have entered the correct credentials", MsgBoxStyle.Information)
            Else
                ' if credentials did not match
                MsgBox("You have NOT entered the correct credentials")
            End If

        Catch ex As Exception
            'If user never registered or something bizarrly happens
            MsgBox(ex.Message, MsgBoxStyle.Information, "Error")
            Exit Sub
        End Try

    End Sub

    Private Sub button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        createUser(UsernameTextBox.Text, PasswordTextBox.Text)
    End Sub

End Class