Windows 7 owners will be able to downgrade heir operating system to Windows XP and not only Windows Vista if Microsoft offers the according contracts with PC manufacturers.
Microsoft Campus in Redmond
Microsoft confirmed that people who buy a new PC will be able to downgrade their operating system to Windows Vista and even to Windows XP. The OEM partner HP was the one to get the ball rolling when the company announced that they want to offer a Windows XP downgrade.
Talking to Mary-Jo Foley a Microsoft representative confirmed that the downgrades will be offered by all OEM producers that participate in the program. But this would be up to negotiations between the participants and not an overall promise. Microsoft would be offering Windows downgrades to the direct predecessor only for the first time.
As an example customers who decide to take a volume license. In this case customers could buy the package and downgrade even to Windows 98 or Windows 95 if necessary. Between October 2009 and April 2010 HP wants to run the downgrade program, but this hasn't been officially confirmed yet.