Problem with Silverlight MediaPlayer

Dear all.
I'm write a simple asp.net code to run video with silverlight mediaplayer
<asp:MediaPlayer runat="server" ID="Media1" ScaleMode="Stretch"
                Height="344px" Width="460px"
                SilverlightBackColor="white" MediaSource="~/lake.wmv" MediaSkin="Simple" />
In first. It's work well in win XP. However when I setup new OS (vista). It isn't work.
I'm currently using VS2008, silverlight beta 2 for VS 2008.
Please help me to solve this problem.
Many thank. 
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