Microsoft has a program assisting businesses who buy PCs with Windows Vista Business (or Ultimate), but want to run XP on them until their software is fully compatible with Vista. Businesses now have downgrade rights, which allow owners of fully licensed Vista PCs to install Windows XP over Vista completely for free, without having to own an unused copy of Windows XP.
According to the program PDF, computers with licensed OEM (comes with a new PC) copies of business editions of Vista can install a copy of XP, one that they must provide themselves. Once XP is installed, the operating system will fail product activation, as it should, since it is a previously used unlicensed copy of XP. Call the Activation Support Line, explain you are downgrading, and they will give you instructions on how to activate it, apparently completely for free.
This doesn’t just work for Vista, it also works for downgrading XP to Windows 2000. In all cases, the business must provide the operating system install disks themselves, but they can be previously used versions without product keys, and only business-class versions may downgrade to other business class versions.
The downgrade paths allowed:
According to Mary Jo Foley, Microsoft is working on simplifying the process further, so businesses can submit batches of keys, instead of calling for each one.