Users who purchase new PCs installed with Windows 7 Professional and Ultimate will now be able to downgrade to XP up until January 15, 2015 (for Windows 7 Professional) or January of 2020 (for Windows 7 Ultimate), according to Computerworld’s calculations. Originally, downgrade rights — the sanctioned ability of Windows volume buyers to apply their new Windows licenses to older versions of the product — were due to expire around 2011 for Windows XP.
From Microsoft's “Blogging Windows” blog:
We have decided to extend downgrade rights to Windows XP Professional beyond the previously planned end date at Windows 7 SP1. This will help maintain consistency for downgrade rights throughout the Windows 7 lifecycle. As a result, the OEM versions of Windows 7 Professional and Windows 7 Ultimate will continue to include downgrade rights to the similar versions of Windows Vista or Windows XP Professional. Going forward, businesses can continue to purchase new PCs and utilize end user downgrade rights to Windows XP or Windows Vista until they are ready to use Windows 7.