How to downgrade Vista to XP

Hi Everyone,

Here's what we have been doing to deploy XP Prof. PC's. Setup a new PC with
all the required apps and settings, sysprep it and take the image using
ghost. The image would be copied to all new PC's and during the Mini Setup
you would be prompted for the XP license key. I would have purchased OEM XP
licenses and a new key would be inserted at this point. All worked just
fine.

Until, I can no longer buy anymore XP OEM Licenses.

I'm told I can purchase Vista Business OEM Licenses and downgrade them to XP
Professional.

Given the way we are presently deploying our XP machines what are the steps
required to use our existing XP images and to have the Vista OEM Licenses
downgraded? I'm sure it's just not a matter of using the Vista OEM License
key in place of the XP OEM License key during the Mini Setup, or is it?

TIA

Greg