The only word of caution on putting retail XP back on a laptop is that some things may not work e.g. sounds or even possibly video (beyond 640x480 or 800x600 in 16 colours).
So one has to google and hunt down the drivers.
This is normal for people building desktop PCs but laptops normally come with an appropriately customised version of XP with all the drivers.
Tip: often, a laptop will have a "drivers" directory on it somewhere, and even if it comes with vista, that directory may contain XP drivers.
I wouldn't touch vista with a bargepole - too much stuff doesn't work under it, including the only IFR flight planning software (Jeppesen).