Can not play asf file

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WinXP

Windows

VisualC++

Hi ,
Now I am making a little stuff that can capture screen frames ,write them into asf file,I use windows format sdk as muxer(use compressed data),and windows media video encoder 9 DMO as coder,the method of writer object in wmfsdk WriteStreamSample return S_OK and DMO's method ProcessInput and ProcessOutput boths return S_OK,generate .asf file successfully ,but i can't play it ,I drag it in AVICodec ,it says "Windows Media Video 9, Not checked",What is that supposed to mean.thanks for any help.