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.
@ScottChamberlain Just an FYI, I often consult this post when I need the ISOs; these links are dead now. – cutrightjm – 2015-03-30T15:13:13.447
@ekaj Thanks, I added a warning about it in the post. – Scott Chamberlain – 2015-03-30T16:29:02.910
2Thanks for your answer, Scott. We would prefer it however if instead of just providing a link, you included a bit more of the information on the page. Our goal is for users to be able to get the answer here when possible. Thanks! – nhinkle – 2011-04-18T00:42:47.477
@nhinkle, I updated my post to be a bit more descriptive of what the link is and where they got the data from. – Scott Chamberlain – 2011-04-18T00:47:56.657
For other language versions, you can find them on p2p (be sure to check the sha1 afterwards with MSDN/Technet – faket tend to crop up these days…) – kinokijuf – 2012-01-13T16:56:03.090
Is it a good idea to link to MDL? – Synetech – 2012-02-09T04:21:36.273
@Synetech a friend was chatting with a Microsoft rep online, and the rep pointed him to these links to download - so I assume it's safe if Microsoft is pointing people to them – cutrightjm – 2014-01-09T04:21:59.677