Why you would want to is beyond me. It seems to work fine for gamming on my bros PC which is not even top range compaired to modern PCs.
You basically have to scrub your entire HDD if I am not mistaken, but I am not sure that gets you to install it. Try setting it to load from CD in the BIOS over from HDD, also try inserting the XP disc and running it while the OS is loaded.
Never had to downgrade OS, so this is pretty much hit or miss guessing.