Is this a store purchased, pre-built computer (Dell, Acer, HP, etc...) or a home built computer you put together yourself?
If you built it, you should have the XP installation CD. If it's a store purchased computer, it should have either came with it, or given you the option to create it from a hidden partition on your PCs hard drive. If it's a store purchased computer and you do not have a Windows XP installation disk, you can try and contact the computer manufacturer for a copy (usually at a cost).
If it's a computer you purchased yourself, well, I'd probably recommend an OS upgrade.
-Wolf sends