I bought a new computer end of August with Vista Home Premium. For another $100-200, could've bought a similar computer with Vista Business, along with a downgrade disc for changing to Windows XP.
At the time, it seemed ridiculous to pay MORE to DOWNGRADE.
But now I regret not having done so.
The problem you may have if you install XP, and the reason I'm not doing it here, is that you may lose some functions on your computer which the computer manufacturer integrated with Windows Vista.
(The computer which came with the downgrade disc was apparently set up so its functions would work with both systems. But that's not the case with the computer I purchased - or at least I was told I'm taking the chance of losing some functions).