There is a thin line between Windows Vista and Windows XP. And users of both versions of Windows can cross it in either direction. From Windows XP to Windows Vista via an upgrade and from Windows Vista back to Windows XP through a downgrade. Sounds logic, doesn't it? Now, I have already addressed the Vista to XP downgrade subject here, but I will bring further clarification.
First off I want to make one thing clear. If you purchased a Windows Vista installation package, or even an upgrade edition then downgrading is not for you. Essentially downgrading is reserved for Microsoft's Software Assurance customers that have acquired Vista via volume licensing. But it is not exclusive to them. Non VM customers can also exercise their rights to downgrade Vista. However, they must actually own a copy of Windows XP Professional.
The beauty of the downgrade process is that, for example, a user that has purchased multiple machines preloaded with Windows Vista Ultimate can downgrade them all with a single Windows XP Professional image. "Per the EULA and OEM DTOS Agreement, the end user customer is responsible for supplying media for the OEM or customer to create the downgrade product image on the customer system. Neither Microsoft, the OEMs, nor the system builders are responsible for supplying this media. VL customers already have the necessary media. For non-VL customers, the easiest way to obtain this media is to buy a single FPP copy of Windows XP Pro or use an existing FPP copy of Windows XP Pro if they already own one. That media can then be used to downgrade Windows Vista Business or Windows Vista Ultimate machines to Windows XP Pro," according to David Overton, Microsoft small business specialist.
While installing the Windows XP Pro downgrade image will not cause any issues, activating it will most likely fail. This is why users will have to contact activation center and specify the fact that they are performing a downgrade. When calling, keep in mind to have the Vista product key close by, as you will need it. In the end users will be able to effortlessly complete the migration to Windows Vista.
"If the customer uses a product key for Windows XP Pro that they've previously activated, that machine will likely fail activation. If the downgraded machine fails activation the customer will be prompted to call the product activation call center to request a key that will enable them to pass activation. The customer should mention that they're exercising their downgrade rights to the customer service representative in the activation call center," Overton added. More information here and here.