Male a Screen Recorder Error-Help Please :D

Well i want to make a screen recorder that takes a screenshot evrey so often and saves them all to an AVI and i made this, but wen i try and open the AVI its not recognized:

Code:

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Imports System.IO

Public Class Form1
    Dim Screen As New Bitmap(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
    Dim Screensize As New Size(My.Computer.Screen.Bounds.Width, My.Computer.Screen.Bounds.Height)
    Dim G As Graphics = Graphics.FromImage(Screen)
    Dim Stream As New FileStream("C:\TestVideo.avi", FileMode.Append)

    Private Sub BtnStart_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnStart.Click
        TmrCapture.Start()
    End Sub

    Private Sub BtnStop_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnStop.Click
        TmrCapture.Stop()
    End Sub

    Private Sub TmrCapture_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TmrCapture.Tick
        G.CopyFromScreen(New Point, New Point, Screensize)
        Screen.Save(Stream, Imaging.ImageFormat.Bmp)
    End Sub
End Class

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OK please help :)