[Deleauvive]

I thought of something a bit radical, and really it doesn't comes with any warranty :

You could install a "N" version of Windows Vista (or use Vlite) to have your system setup free of WMP 11. Then may be, there would be less conflicting files when installing WMP 10.

That's if you really can't live without WMP10, but there are other features-packed free Media Players available, The KMPlayer being an example of such.

Again, reread the whole thread and make sure you didn't miss any step.