"The original key was stuck to the case" that sounds like OEM to me. Are you sure they are Retail? AFAIK Microsoft opted not to supply Media Centre as an independent retail version. You can check what you have and if CD is OEM by looking at the (product identification) Pid = number in the i386\SETUPP.INI configuration file (open .INI files with notepad) after the number it will probably say OEM.
Post back when you have found out as all may not be lost and I'll say which files to look at in notepad from the CD to check.
To explore the contents of CD (press and keep shift key held down while you insert CD in tray and until CD drive activity stops - this prevents the CD from autorunning) then look at contents in Windows explorer.