This applies to Windows Vista Ultimate and Window Vista Business only - no other versions of Vista. If Vista Ultimate or Vista Business came with your computer - OR you bought the full retail version of those (NOT an upgrade version), OR your organization has a Volume License Agreement using one of those versions or Windows Vista Enterprise, you can downgrade to Windows XP Professional or Windows XP Tablet PC edition if you want. Here are the rules:

• You cannot use the copy of Vista on another computer - if you downgrade a computer to XP, no fair using the copy of Vista on another computer.
• You must supply your own installation media. You can use Volume Licensing media (provided the End User has a Volume Licensing agreement), Retail (Full Package Product), or a System Builder edge-to-edge CD (OEM) and must supply your own valid Product Key.
• You will probably have to provide your own support or pay for a Support Incident from Microsoft should you require support (”neither Microsoft nor the PC manufacturer is obligated to provide customer product support”).
• On a computer that came with Vista, you cannot remove Vista from it and then install XP on a different computer.
• You may reinstall Vista on the computer you downgraded, provided you remove the copy of XP.
• I’d check the licence to be sure, but it sounds like if you want to multiboot Vista and XP, you must have a separate licence for XP - you can’t use the downgrade right to install a copy of Vista and XP on the same computer.
• Product Activation will probably fail on the downgraded system because the Product Key has probably been used on another computer. Telephone “the Activation Support Line and explain the circumstances to the Customer Service Representative. Once it is determined that the end user has a valid Vista Business or Ultimate license, the Customer Service Representative will help you activate your software.” In other words, keep your Vista Product Key handy when you make the phone call.

Downgrade rights also apply to Small Business Server 2003 Premium Edition, some versions of Windows 2003 Server, and some versions of Office. Here is the link to the downgrade chart and a FAQ. The terms for OEM software (software that was installed on your computer when you bought it) are on the System Builder Web site, which is only available to registered System Builders, so I’ve reproduced them below.

• The right to downgrade is an end-user right and is documented in the OEM System Builder product end-user license agreement or EULA.

• OEM System Builder Downgrade Rights for operating systems apply to Windows Vista Ultimate, Vista Business, Windows XP Professional, and Windows XP Tablet PC editions only. For Server, these rights apply to only Windows 2003 Server Standard and Enterprise Editions and Windows Small Business Server 2003 Premium Edition. OEM versions not mentioned do not have downgrade rights.
• If the End User chooses this option, it is noted in the Product EULA that neither Microsoft(r) nor the Manufacturer will supply such End User with media for such previous versions. The End User must provide the downgrade media and a valid Product Key.
• End Users can use the following media for their downgrade: Volume Licensing media (provided End User has a Volume Licensing agreement), Retail (Full Package Product), or a System Builder edge-to-edge CD (provided software is acquired in accordance with the Microsoft OEM System Builder License). End Users can use one legal version of the downgrade media for multiple installations. The end customer does not need to have one set of media for each PC they are downgrading, because they have acquired legitimate full operating system licenses for the most recent version, which gave them the right to downgrade.
• In the event the End User opts to downgrade, neither Microsoft nor the PC Manufacturer is obligated to provide customer product support for Customer Systems on which such previous versions are installed.
• The software may not be installed on any other computer system than the one which was downgraded under this right.
• If downgrading the End User cannot use the downgrade operating system (for example Windows XP Professional) and the most recent version which they downgraded from (for example Windows Vista Business) at the same time.
• If downgrading Windows Server 2003 or Small Business Server 2003, the End User cannot use the downgrade server software and Windows 2003 software at the same time.
• If an End User chooses to downgrade to an approved, prior version of the Windows desktop operating system, the PC Manufacturer is still required to apply the desktop operating system Certificate of Authenticity (COA) to the PC.
• End Users may reinstall Windows Vista Business, Vista Ultimate, Windows XP Professional, Windows XP Tablet PC edition, Windows Server 2003, or Windows Small Business Server 2003 at any time, provided the downgrade operating system has been removed from the computer, and that software is reinstalled on the PC it was originally installed on with the original OEM System Builder edge-to-edge CD distributed with the original PC. The End User will need to use the Product Key, located in the center of the Certificate of Authenticity (COA), in order to activate the product.
• When an end user is using their downgrade rights offered under the License Terms in Windows Vista Business and Ultimate versions and they use both Windows XP media and a product key that was previously activated, they will fail activation due to the hardware configuration change when installing on the Vista system. In these cases the end user will be prompted to call the Activation Support Line and explain their circumstances to the Customer Service Representative. Once it is determined that the end user has a valid Vista Business or Ultimate license, the Customer Service Representative will help them activate their software.
• For Client Access License (CAL) downgrades, end users may use Windows Device CALs and Windows User CALs with an earlier version of the software in accordance with the terms of the of the Windows Server 2003 EULA; however, due to technical incompatibilities, the end user may not use TS Device CALs with TS User CALs.

[tags]vista, client access license, CAL, downgrade to XP[/tags]