Are you wanting to get rid of that Windows partition? If so, then just install Jaunty to it using only a / ext4 partition. If it's just for testing Jaunty out, then there is no need to separate a 30GB partition into various partitions. I would imagine that Jaunty will recognize Hardy and adjust the /boot/grub/menu.lst file accordingly. However, the MBR will be written to your new partition. If you want to avoid that, then just decline that GRUB be installed during the Jaunty install. Once finished, boot into your Hardy install and edit the menu.lst manually.