Most people would probably say that using WMP to play videos is madness. VLC (as recommended by BBC technology programs and many sources in the technical press) is far better, as is GOM.
However, if you want to get Microsoft's pathetic program to work properly, you'll need to install the relevant codecs. Go here to download the installation file:
http://fileforum.beta...Standard/1094057842/2
Save the file to your hard drive, and then double-click on it to complete the installation. Reboot your computer before checking whether WMP will work properly.
NB: Those instructions mean downloading and running an executable file. Whenever you do that there's always the risk of introducing a virus onto your computer. I've taken care to provide a link to what should be a reputable site but I still recommend running a virus scan before double-clicking on the file to run it. (That's usually achieved by right-clicking on the file and selecting 'Scan with ...', depending upon which antivirus program you've got).
Chris