its akward.
if you are buying new, you can ask the vendor like lenovo or HP distributors doing that for you.
if its not new, you can still try. I don't think its free (about US$15???).
downgrading is legit. vista license covers XP liense.
you can copy a XP key, use XP CD to reload the hard drive. But make sure you have XP drivers for all the components.