IF you buy a new computer with VISTA there is no reason to buy XP as the License explicitly allows you to install XP using the Vista license key.
Rights to OEM versions of systems software are granted in the OEM License Terms. The OEM License Terms for most OEM versions of systems software do not grant downgrade rights. The exception is the OEM License Terms for the Windows® XP Professional operating system and the Windows Vista™ Business and Windows Vista Ultimate operating systems, which grant downgrade rights. See the full text of the OEM License Terms for the specific downgrade rights.
There are so many subtle things that speak of how many problems that Vista has. This OEM License downgrade option is one of them. In so many words, Redmond is saying, "If you really don't like (read "can't tolerate") Vista then you don't have to keep using it. How much plainer need it be said?