Search BT Junkie (BitTorrent) for "XP Lite SP2".
There's a downloadable ISO of Windows XP that's been modified for old computers. With a fresh install, resources are only at like 40MB. However, basically every un-vital service has been removed, and it comes with basically NO additional programs. You'd better have all your drivers on a disk too, 'cause most of the drivers have been removed as well. Full install comes out to about 200MB.
However, it's probably illegal, if you're not into that kinda' thing...