To setup a multi-boot system.
1. you need to have a unformatted partition ready for the new distro to go into.
2. When you install the new distro, do not install the bootloader to the MBR, install it to the root partition or the superblock.
3. When you reboot, you will not see your new distro on the list, you will need to add it to your current bootloader.
A. Open up a terminal, type su and your root password.
B. Then type "mkdir /mnt/new distro"
C. Then "mount /dev/hda1 /mnt/new distro" modify hda1 to suit your needs.
D. Type "cd /mnt/new/boot distro"
E. Look for the name of the kernel, it should look like vmlinuzxxxxxx, if you see a initrdxxxxxx you will need to add that to the menu as well.
F. Use a text editor to open menu.lst or grub.conf. Hopefully you are not using Lilo, the next few directions won't be correct.
"nano -w /boot/grub/grub.conf"
title new distro
kernel /boot/vmlinuzxxxx root=/dev/hda1
Hit CTRL + x and save your work
Reboot and finish setting up the new system.