I also tested on Windows XP (should have mentioned this). All the PCs I tested were running XP, except my home computer where I tested both XP and Win 7.

Note that under Win 7 I could see the window rendering in the preview on the taskbar, but the actual window does not show. Due to this I conclude it renders ok, and DirectDraw works, but for some strange reason it the actual window is not visible. Also, the fact that the same problem is exhibited on XP means it's probably not Win 7 related.