Not going to happen, I'm afraid. The C2D and i7's require different motherboard slots and chipsets, so you cannot simply swap one CPU for another. Laptop motherboards are not interchangable (due to each laptop being a different size, shape, and port layout) so the chances of finding one that will take an i7 AND fit into your laptop case AND allow every connector to line up is virtually nil.
The only hope you would have is if Alienware produced your model laptop in C2D and i7 versions, then you might be able to swap the innards from the i7 version into yours!
Best advice, do you NEED an i7? When was the last time you were limited by the CPU in your laptop?
If you definitely want/need an i7 then sell your laptop and buy an i7 one...