Two editions of Windows 7 will be eligible for downgrade rights: Windows 7 Professional, and Windows 7 Ultimate. Windows 7 Professional buyers will be able to downgrade to Windows Vista Business or Windows XP Professional, while Windows 7 Ultimate buyers can downgrade to Windows Vista Ultimate or Windows XP Professional. Downgrade rights will be granted for 18 months after Windows 7's launch, so from October 22, 2009, to April 1, 2011.
Per standard policy, organisations with volume licensing deals with Microsoft will be able to downgrade to whatever version of Windows they please, at any time. Volume licensing deals are for organisations with 250 PCs or more.
There is one possible catch: downgrade rights to Windows XP will expire before 18 months if a service pack is shipped for Windows 7. Downgrade rights to Vista will not expire, service pack or no.
Windows XP is no longer available for normal computers or at retail; only netbook manufacturers have access to Windows XP Home, but only until one year after Windows 7's release.