video problems with windows media player

Whenever I try to watch a music video using windows media player, all I get is only the sound with no video. Then I receive the following message: ” Unable to download an appropriate decompressor ” What is the problem and how to solve it?

My computer runs windows 98.
I have got windows media player7 downloaded and it causes me no problems but still some music videos are displayed through windows media player which gives me the mentioned above message. Why couldn’t all music videos be displayed using windows media player7?