As described in this Microsoft KB article, if it is a retail copy, you have the following recourse:
To request replacement hardware or software, contact the Microsoft Supplemental Parts team at the appropriate contact number that is provided in the "Contact Information" section. You must be in possession of the product (if it is damaged or defective) and be able to provide proof of purchase. The customer service representative will advise you of any replacement costs or shipping and handling fees that may be charged. If a product is no longer under warranty, the replacement item may no longer be available.
If it is an OEM copy, then they advise you to contact your OEM for the replacement disc instead.