Mydigitallife reports a more advanced set of commands to set 64-bit WMP as the default player in 64-bit versions of Windows Vista x64. If the simple unregmp2.exe /SwapTo:64 command doesn't work, try the following:
"To switch and change to x64 or 64-bit of WMP11, open an elevated command prompt as administrator and type the following command:
Then, open Registry Editor (regedit), then navigate to the the registry key of “HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths\wmplayer.exe”. Then set the registry subkey “Path” to be “%Programfiles(x86)%\Windows Media Player” (without quotes), and “(Default)” subkey (normally the first entry) to “%ProgramFiles%\Windows Media Player\wmplayer.exe” (include quotes).
To undo the x64 change, switch, swap, revert and change back the default media player to Windows Media Player 11 x86 32-bit edition, also open an elevated command prompt with administrator privileges, but issue the following command instead:
Then, run Registry Editor, and reset the value for registry subkey of “Path” and “(Default”). With either one of two methods, you will have complete full x64 Windows Media Player experience."