Can't decode B frame while playing mpeg4 bistream

Dear all,
    There's a problem while playing mpeg4 bitstream. I use VirtualDub with
SigmaDesign rmp4 CODEC to encode a video sequence. But in my PC, while I use
win2000's mplayer to play, there's no B frames decoded. But the other PC can
decode the B frames. Are there any dll or other files I should remove for
mplayer to decode the B frames? I install some software for MPEG4. I don't
know if they are related to this problem. Thank you!~~