The issue may well be that you have an improper codec being used.
You can download a multi-purpose codec (FFDShow) which will decode a variety if internet formats perfectly .. including DVD, XVid, and some Real and Quicktime formats.
Very small .. and is not a program .. just the 'spanners' to make formats display. Try it first before anything else. It can stay on your system for use playing formats that Windows would otherwise not display.
http://www.free-codec.../download/FFDShow.htm
There is a x64 bit version there if required (Check your pc to see if x64 bit operating system) Install it and reboot .. try again and come back!