they won't do it
They don't even do it now for Vista. I have tried several times to get Microsoft to give me the XP code for proper activation after downgrading and they WON'T do it. They offered to do it for a fee of $60.00 once. Here's what Microsoft says they WILL do (but won't).
"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 be unable to activate on-line over the
Internet, 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."
Here's the full pdf of what Microsoft says your downgrade rights are (but they ignore it).
download.microsoft.com/download/5/f/4/5f4c83d3-833e-4f11-8cbd-699b0c164182/royaltyoemreferencesheet.pdf