Even if you can get ahold of Windows XP you better make sure you can get XP drivers for your system. I tried to get XP to install on my Win 7 Toshiba laptop - Toshiba only has Win 7 drivers for my model.
XP installed fine but no sound, no webcam, only base VGA video (no HW acceleration or DirectX/3D) and no SD card reader support. Device Manager was full of question marks.
I was able to get XP video drivers for my Radeon video from AMD's website but can't get Conexant HD sound drivers to work. Trial and error with countless drivers to finally get webcam to work but still far too many questions marks in Device Manager for my liking.
This is what you will be likely facing if you try to get XP working with modern computers and the vendor has no XP support. Hours of Googling, reading through endless threads in user forums, hunting down and trying to download hacked drivers from years-expired links and finally crossing your fingers that they work.
If this is your idea of a fun Saturday afternoon you're a better man than I.
HP, Dell, Gateway and Toshiba's stance is basically what your system came with is what will be supported driver-wise and nothing else. Go to their user forums and see for yourself - you want to put XP on your Vista/Win 7 machine you're on your own, they won't help you.
Not sure about ASUS, Lenovo.