You need to have Pro based version, not a Home one, and it had to have come pre-installed on your computer. You lose the original license, and have to format your drive, but they still call it in EULA terms an 'upgrade'. There appears to be no real upgrade.

The page also makes it appear to be a very driver-light release, but I don't have a copy so I can't say for sure. It also does a very un-MS like thing and absolves the OEM of support responsibilities while giving you a single free support call.

All of this costs $12 shipping and handling, $22 if you are out of the US. You can also check out a few more pages.

Homepage: http://www.microsoft.com/windowsxp/64bit/default.mspx

Order ($12): http://microsoft.productorder.com/clientx64/

Why: http://www.microsoft.com/windowsxp/64bit/evaluation/top5.mspx

Compare: http://www.microsoft.com/windowsxp/evaluation/compare.mspx