Windows Media Player comes broken on 64-bit Vista
Well, does that really come as a surprise? There really is no advantage to running a 64-bit (Windows) operating system yet. Driver support is horrible, some programs don’t work and generally it just makes life worse. And that friends, extends to the 64-bit version of Vista.
It seems 64-bit Vista has a bit of trouble with Windows Media Player (WMP). But not just Media player, this problem can extend into other programs as well such as Napster. By default the 32-bit version of WMP 11 is set as default on a 64-bit Vista installation but sometimes the 64-bit version of WMP will try to take over control resulting in an error.
Full article here: Source