We got two new laptops for home last week from Dell Business, one loaded with XP and one with Vista. So far, so good with Vista, its sort of XP with prettier graphics once you have turned off the extra user control thingy. However, when I was searching PC World certainly had XP laptops available in their business centre online pages, a straight forward product search brought them up. As for processors, not really up to speed these days, but like all things, go for the best you can afford. Dual core are the most commonly available it seems these days and my desktop runs an AMD 64 with XP 64bit incredibly well for 95% of stuff, just a few things that wont install because of the 64bit functionality, usually free stuff like Ad Aware.
Pretty sure the techies round here can better advise on processors but I can definately say XP machines are still available but in most cases you will probably have to order them online and pay a bit more for the older OS than Vista. The Dell came with a Vista upgrade disc for as and when we want to upgrade, but I think Dell will only sell these to businesses, their home range is all Vista with no option for XP :bonk: