See this link:

[url]http://asia.cnet.com/crave/2007/09/24/downgrade-your-vista-to-xp/[/url]

The deal seems to be that some OEMs will
provide you with a downgrade. If you can't
get that, and you're willing to call Microsoft,
the article seems to be saying that MS will give
you the necessary activation code if you can
find an XP CD to install with. If you go that
route, I'd call the OEM first. Failing that, check
with MS before proceeding.

That all only applies to the Business version.
For Home version, the cheapest legal way is to
buy an OEM version disk (about $100 US) online.
I like buycheapsoftware.com. Their name sounds
hokey, but in my experience they're actually very
dependable and have good service.