If this is a fairly new laptop and came with Vista already installed then to be honest I would stick with it, and as others have said, add more memory, which is VERY easy.
If it is an older laptop that originally came with XP (and was later upgraded to Vista) then you may have the original XP disks so you can re-install.
As others have said, drivers are the main issue. If you have never installed an operating system before (and you sound as though you havent) then you may not understand the importance of drivers.
Every piece of hardware inside your laptop needs a driver (sound, graphics, networking, other things you may never have even heard of).
AFTER you install an operating system you then need to install the drivers. If you dont then you may find than many things on your laptop do not work.
If it is a NEW laptop it may be that nobody has ever written XP drivers so you may not be able to get these things to work at all under XP.
To be honest if you dont know what you are doing you are entering a bit of a minefield.
Just add more memory (have at least 1Gb, although 2Gb would be best) and it will solve all your problems.