To help smooth over the path to Windows 7 upgrade nirvana early corporate adopters were given a cop-out in the license agreement to run Windows XP instead on the associated PCs. That grace period will soon be ending meaning those with specific reason to keep using XP had better get a move on.
The reason behind this convoluted idea is explained in the Microsoft licensing docs here as a way for users to upgrade PCs early and get a copy of the new OS but keep using XP until they were comfortable and ready to upgrade from XP. Basically an effort to get the upgrade cycle moving for disgruntled hardware makers fed up with slow Vista-based system sales.
Follow Microsoft’s Yellow Brick Road when it comes to upgrades
Only certain OEM versions of Windows 7 include downgrade rights specifically Windows 7 Professional and Windows 7 Ultimate temporarily include the right to move back to Windows XP Professional Windows XP Tablet PC Edition or Windows XP x64 Edition. There is a PDF explaining the full route here.
The end of the offer will most likely kick in whenever Windows 7 Service Pack 1 kicks in sometime in the next few months. After that enterprise users will only be able to downgrade in some specific circumstances to Windows Vista Business. Microsoft already extended the deadline for downgrades once last year but is unlikely to do so again.