Hmmm....

Well, instead of using DirectX 8.1, you should use DirectX 9.

Did you replace the ddraw.h in DirectDraw 4 with the one for DirectDraw 9?

I looked at my own DirectX game, and found this:

IID_IDirectDraw7

Seeing as how it's case sensitive(or at least that's how my compiler works), you should try uncapitalizing the middle of the words.