Transparent WPF window blinks when background is redrawn

Hello everyone.

I have a problem with a window which is transparent. If the background changes under the window, you can see that the whole window is redrawn, so it blinks and it doesnt look well.

<Window x:Class="Transparent" WindowStyle="None"
AllowsTransparency="True"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Transparent" Background="Transparent" Width="190" Height="200"
Topmost="True">

  <Canvas Background="#FF444444" Height="200" Width="190" Name="TranspaerntCanvas"></Canvas>
</Window>

On some changing background it works fine (for example when I put it over a youtube video), but over openGL background the window is blinking. I use .NET 3.5 on win XP.

Does anyone know, how to fix this problem?

Thank you