Your account may have administrative rights, but, in Windows 7, you will not normally be running as administrator.
I can't think why you would wish to erase the Windows Media player installation; repairing the installation (which is, pretty much, part of your Windows setup) would seem to be a better option. I therefore hesitate to advise you, as the advice could turn out to be very unhelpful. Please could you describe in detail what you are trying to do.
I am not sure, but I could readily believe that the WMP installation contains files that users are just not permitted to access; no application (including Eraser) can touch these.