If you own a NTSC Xbox, you'll need the NTSC game-disc, afaik.
Does your XBox work on European TVs or is it connected to a HDready/FullHD Plasma/LCD?
If it works on PAL TVs, perhaps it can actually read PAL discs.
Let me clarify: I haz the x360 slim ntsc, and It plays on Pal Tvs. For example, I already have the Pal version of Portal 2 and it works just fine, but that game is region free so it doesn't matter.