You might get away with transferring the hard drive from the old to the new laptop, but you will probably have some "challenges" along the way, in particular for the video and sound drivers. If there's any "manufacturer proprietary" stuff on the old laptop, that may get the hump, too. The XP installation may complain and ask you to revalidate - not a problem if you have the COA and documentation.
If that doesn't work, I'd be inclined to buy a new laptop (without an operating system, unless you plan to keep the old one), install the XP etc from your present laptop, then copy your documents etc across.
You could even buy a HD caddy, and put the old machine's HD in that, to use as an external drive for the new laptop.