In Media Player, First, click "Tools" --> "Options".

Second, click the "Network" tab, and highlight the MMS proxy setting as shown below.  Click configure below that.  If the box is checked that says, "Do not use a proxy server", change it to "Autodetect a proxy server."  You should be able to play the videos now.

Explanation:   This setting has been changed because of some of the latest Windows updates such as IE6 updates and the new IE7 download.