I've had the very same problem before, and it wasnt the CD that has problems either (not saying that this is 100% the case with you) but certinally I wouldnt be getting a new CD just yet.
As the above poster suggests, it might be a problem with the HDD or Memory.
Try re-formatting the drive and starting again as the first thing to do.
And as to your actual question, i dont see why you wouldnt be able to get a new CD from MS, perhaps for a £10 admin charge or whatever. You own a license to use it, so i would hope that they wouldnt charge you much more than the cost of a CD and delivery assuming you can prove that you have the original.
Hope it wont come to that anyway