I don't know about banning it, but it's gonna take some time before my employers make the switch to Vista. They've only just finished switching everyone over from NT to XP....they think. We had an IT guy coming round the other day doing an audit of what they've got and what they're running. Same thing about six months ago.... *sigh*
I've a friend who's looking to buy a new PC, and after I told her "upgrading" to Vista right now probably wasn't wise, she's now wanting one with XP and doesn't want to pay any more than £300. She reckons everywhere will be trying to sell off all their stock with XP on it cheap. Of course dopey me gets roped in to finding her a good deal. Of course everything's shipping with Vista pre-installed now. After a couple of hours searching I found one at less than £300. 512MB RAM....no good. She wants at least 1GB apparently.
XP + <£300 + 1GB RAM. Chances not good.

I've suggested trying local computer shops where they build according to your specs.