windows media player video playback problem?

[Sidewinder]

For the past few weeks, I've been having a weird problem with windows media player. When I play a video, the playback starts out fine, but after some time, abruptly, the video and audio lose synchronization. The audio plays on at normal speed, but the video playback is slowed down considerably. The problem occurs most of the time I play videos. The exact time at which the problem starts is also random. Sometimes, it occurs in the first minute of the video, sometimes its the 17th minute, sometimes its at the 23rd minute..etc.

Can somebody identify what is going wrong?

Thanks for taking the time.