I have had the following question asked to me quite often so I figure it was worth posting:
Can I downgrade my OEM version of Microsoft Windows Vista Business to Windows XP Professional?
Yes. OEM Downgrade Rights for desktop operating systems apply to Windows Vista Business & Vista Ultimate as stated in the License Terms. Please note, OEM downgrade versions of Windows Vista Business and Vista Ultimate are limited to Microsoft Windows XP Professional (including Tablet PC Edition & x64 Edition). End users can use the following media for their downgrade: Volume Licensing media (provided the end user has a Volume Licensing agreement), Retail (FPP), or System Builder hologram CD (provided the software is acquired in accordance with the Microsoft OEM System Builder License). Use of the downgraded operating system is governed by the Windows Vista Business License Terms, and the end user cannot use both the downgrade operating system and Windows Vista Business. There are no downgrade rights granted for Microsoft Windows Vista Home Basic or Home Premium.”