Windows 7 operating system supports both Windows Media Player 32-bit or Windows Media Player
64-bit or in other words, any version of WMP 12. But in order to provide maximum compatibility and codec support, Windows Media Player 32-bit versions has bee used as the default Windows media player. On the other hand, Windows 7 locks the system registry key and does not allow users to manually make WMP 12 as default media player.
WMP 12 64-bit (x64) can run the video with more exposure as it uses architecture advantage of x64 operating system and runs faster than the other flavour of Windows media player. Windows Media Center uses 64-bit WMP as it’s default player and from this point you can consider it’s productivity features. In order to make Windows Media Player 64-bit as default player in Windows 7, we need to install few video codecs and make some changes to unlock the registry key.

While browsing, I found an excellent tutorial which can be found here. This tutorial tells the detailed how-to steps to switch from Windows Media Player 32-bit to Windows Media Player 64-bit to make the default video player in Windows 7.