"Class not registered" is one of the most annoying and frustrating error message I ever got because none of the video files can be played by Windows Media Player, Media Player Classic or other DirectShow player and common fix methods (Windows Media Player reinstallation/upgrade, codec reinstallation...) seems to fail.
Fortunately, the solution (in fact one of them because there are few different solutions) for this problem is very simple.
After some research I found that quartz.dll (a DirectX related DLL) wasn't registered correctly.
How to register it? Click on Start > Run... > and type regsvr32 quartz.dll
In the end, you should receive a message saying that your file was succesfully installed.
Note: Please, let us know if the solution suggested here wasn't helpful for you so we can try to complete this guide with other hints.