There actually is no such thing as DirectX 9.0d, only the frequently updated DirectX 9.0c releases.
(it is quite odd that some sites indeed list the non-existing DirectX 9.0d as a min. requirement for Sacred 2)

V9.0d was expected after the initial V9.0c Release but never came, instead MicroSoft opted for a new, systematic update strategy of 9.0c.

Installing the latest 9.0c Version will satisfy all your DirectX needs (the Game will actually contain an older version of 9.0c), but it won't help your Key Validation problems.

I can only recommend to identify the source of your Key/Media mismatch and maybe borrow another media from a friend that matches your key (e.g. XP Home vs. Professional vs. Server Edition). As the COA Sticker on your Case is a valid license, this should be absolutely legal (e.g. some PC manufacturers don't even deliver an Install Media and require the User to create & burn one himself off the running Installation using an OEM Utility).

Other than that, you can get XP original Media & COA for relatively little money off EBay (just make sure you're buying from a trusted and reliable seller, e.g. established PowerSellers that regularly sell large amounts in order to avoid falling for a black market copy)