I recently read on the internet that the 64bit versions of Vista, dont run the 64bit of Windows Media Player by default, all mainly to do with compatability issues.
If you are running a 64bit machine you can alter the system to run the 64bit version if you so desire, although i tried it and didnt see any improvement, but then didnt really expect to with such an application!
If you do want to go 64bit.
Type in 'Run' and hit enter...
Type %windir%\system32\unregmp2.exe /SwapTo:64 into Open:
This should now switch to 64 bit! To go back again do the same thing but change to...
Like i said, i didnt see any improvement, but when it comes to the trick where Windows Live Messenger can show what your listening to as part of your name, it seems to stop working, which i would put down to the compatability issues as i doubt WLM is 64bit! So if you like that bit to be active then stick with 32bit!