Due to a missing or corrupted required file Windows Media Player may stop working. It is bit difficult to solve such issues, especially for beginners. Here is a freeware, portable tool to repair WMP 12.
Fix WMP from Thewindowsclub will re-register all the concerned Windows Media dll files required for the smooth functioning of Windows Media Player.
It will re-register over 40 dll files required for the smooth functioning of WMP. You can download Fix WMP for Windows XP, Vista and 7 from here.