Windows 7 downgradeable to xp
 

Microsoft Windows 7 Will Allow Downgrades too
Even though there's still some time left before the official release of Windows 7, Microsoft - the creator of this OS - confirmed today that just like some Windows Vista distributions could be downgraded to Windows XP with ease, the same will apply to Windows 7. General users of Windows 7 will be given the option of downgrading right over Vista to Windows XP, a Microsoft spokesperson has confirmed.
"This is not the first time that Microsoft has offered downgrade rights to a version other than its immediate predecessor," the spokesperson told Betanews, "and our Software Assurance customers can always downgrade to any previous version of Windows."
Microsoft hasn't detailed exactly how downgrade rights will work with Windows 7 - for now we know that Windows 7 will be downgradable to either Windows Vista or Windows XP - the rest of the details remain unclear.

Source: Betanews