Where can I find an unused Windows XP install CD?  I looked on Amazon and they're nuts.  An old computer system for over $100 is ridiculous. 

The computer in question is a homemade deal so I can't use one of the 10,000 Dell CD's that are on eBay. 

It has to be an unused one and from looking at a lot of them, they've been re-wrapped making me suspicious that Microsoft will not recognize the key because it's been previously used.

Upgrading is out of the question, the computer just holds data pretty much and worked fine until recently when HD crashed.  Data is safe but it needs to have an operating system for the access program.

Lots of reasons why this is the only way that will work.  Suggestions for other options are no doubt considerate and thoughtful but of no value in this instance.