There is no BootCamp Assistant for Windows XP.
BootCamp Assistant is the application in OS X that partitions your drive and allows for the installation of Windows and makes it bootable.
BootCamp Assistant is not available for download, it would be a part of the original install of 10.5 or 10.6.
BootCamp, the drivers, are on your OS X disc, either Leopard or Snow Leopard.
Once you have installed these drivers from the OS X disc in Windows, then you would use the BootCamp icon in the system tray to check for any available updates.
If you have not done so, I strongly encourage reading the instructions available from the first screen of BootCamp Assistant.