Windows Media player error after OS update

Hello,
I formatted my computer and removed Windows XP and installed Windows 8. After that I could not play most of my media files and Windows Media Player gives me an error.
Is this problem unique to me? Any suggestions?

Windows Media Player cannot play the file. The file may be formatted with an unsupported codec, or the internet security setting on your computer is set too high. Lower your browser’s security settings, and then try again.
