I might be wrong here but my opinion is, if you buy a Vista laptop you wouldn't want to install XP on it as you might have problems. The reason I say this is that there might not be XP drivers for the various parts of the laptop and if you install XP you may well find it wouldn't work properly. I'm thinking specifically about the graphics and things like the touchpad.
This is just my opinion and I could be wrong but I do think this may well be more complicated than doing something like this on a desktop.
I believe that Dell for one will still supply XP if you wish though I am unsure if that do it with laptops as well as desktops but its worth a call or an email.
If you do get a Vista laptop as far as I am concerned the main thing is memory. You need lots of memory, buy a cheap laptop with little memory if you wish as long as you can add more to it cheaply enough. Friend just bought one with Vista Home Basic and 512mb and to be honest its rather slow.