Media Browser and Media Center cannot open file error playing movies

What fixed this for me was going into the Media Browser configuration tool and clicking advanced, and then cache. Clearing the items cache fixed the problem.

Also, depending on the types of files you are attempting to play it could be a codec issue. A good solution to this is the Shark007 codec pack. Here's a link for Windows 7. Scroll to the bottom and you will see the download link to 'Major Geeks'.