The minimal (10Mb or so, startup install) of Ubuntu with XBMC used 31 seconds before it where ready.
Standard "desktop" install of ubuntu (did not remove any unnesesary pacages) used 37 seconds before it where ready.
So, Windows 7 Embeded is a FAST booter
NB: I do not own a copy of Windows XP anymore, or atleast, I cannot find it So I have not XP times to show to.
All times is completely untweaked. So there should be room for improvements.
NB: All boot times is cold boot. No sleep/hibernate.