if you dont mind loosing your current Windows install just boot into the Windows Installer and reformat the partition to either FAT or NTFS and then install Windows again.
after youve installed Windows eject the disc like i said above and then install the Boot Camp drivers from your Leopard DVD.
then to make Mac OS X boot by default open the Boot Camp preferences in Control Panel (while still in Windows) and select your Mac OS X partition then click restart.