What I typically do is install Windows XP with any valid key, then call up the activation hotline. Tell them you're installing a downgrade, and they typically give you an activation code without even asking for your product key - Microsoft really doesn't seem to care about XP anymore. There may be a way to do this that doesn't involve 5 minutes of shouting at the automated phone system to get a live support agent, but I'm not aware of an easy one.
(Disclaimer: I am located in the US. YMMV.)