XP and Vista are nice and speedy on modern to semi-modern laptops (esp. with core 2 duo or centrino duo processors). Mostly, just stay away from AMD processors; they are generally slower and more power-hungry. They tend to round out the bottom-end of laptop product lines.
Dell sells laptops pre-loaded with Ubuntu now:
http://www.dell.com/content/topics/s...=19&l=en&s=dhs