This situation can arise for a couple of reasons, and while the symptoms are similar, the causes are very different, arising from either a firewall or a bandwidth problem. The symptoms for each are as follows:

Firewall:

The video may appear to buffer as normal, but will play only the first frame or so of the video and then freeze while the audio continues as normal. The image may change occasionally, but only every 20-30 seconds.

If this is what you experience, you should refer to I have a local firewall and cannot access the site.

Bandwidth:

The video will buffer as normal and may start to play both audio and video. After a few seconds, the video will pause, but the audio will continue to play. The video will then skip to catch up and the cycle will repeat a couple of times. The default behaviour of Windows Media Player is to renegotiate the streaming rate until it can settle on one with which your local Internet connection can cope. If there is a lack of bandwidth available, Windows Media Player's last action will be to stop streaming video altogether, and only stream the audio.

If this happens, please refer to Content plays but pauses frequently with message, "Buffering".