You most likely need a Codec package for your video player. Codec's are like translaters that take the video image format and encode and/or decode it for viewing in your favourite media player. I would recommend that you download and install the K-Lite Codec pack from here,
Download K-Lite Codec Pack
http://www.codecguide.com/download_kl.htm
Get either the Standard or Full package halfway down the page. There is a package specifically tailored for 64 bit systems if your Windows 7 is that flavour. The 64 bit package is not always necessary as most programs are still 32 bit.
Another option is to download and install a good, free media player that has all relevant codecs preinstalled. If you are not 100% happy with your current video player program, consider,
VLC media player for Windows
http://www.videolan.org/vlc/download-windows.html
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