It sounds like you might not have the required codecs (information for decoding different types of data).
Please go here: http://download.cnet.com/K-Lite-Mega-Codec-Pack/3000-13632_4-10794603.html and download and install this codec pack on your computer.
Please make you have the latest Windows Media Player installed on your computer.
Here is the link to install the latest windows media player:
http://windows.microsoft.com/en-US/windows/products/windows-media-player
Please make sure all important updates are installed on your computer. You might need to run software updates and restart your computer to install all the available update(s).
To update your computer, please note the following:
For PC users: run Windows Update:
Start > 'Control Panel' > ‘System and Maintenance’ > 'Windows Update'