I have Backtrack 4 Beta installed on my hard disk, but I only used one partition for it. Presently, my hard disk has three partitions:

sda1 = FAT16: DOS 6.22 and Windows 3.11 for Workgroups (installed from floppies!)
sda2 = NTFS: Microsoft Windows XP
sda3 = ext3: Backtrack 4 Beta

Before I installed BT4beta, I had Xubuntu installed on sda3. I didn't want to totally wipe Xubuntu off my hard disk, so I did the following:

* First of all, I booted up BT4 off my USB stick, then I did:

cd /mnt/sda3
mkdir xubuntu
mv * xubuntu
This moved my entire Xubuntu installation into a folder called "xubuntu" on sda3.

OK so next I followed the tutorial posted by pureh@te, but I had to alter it for two reasons:
1) I didn't give the bootloader an entire partition to itself
2) I don't have a swap partition

So with BT4beta loaded up on my USB stick I did as follows:

# Note that this is changed from pureh@te's version
cd /
umount /mnt/sda3
mount /dev/sda3 /mnt/bt/
mkdir /mnt/bt/boot
cp --preserve -R {bin,dev,home,pentest,root,usr,boot,etc,lib,opt,sbin,var} /mnt/bt/
mkdir /mnt/bt/{mnt,tmp,proc,sys}
chmod 1777 /mnt/bt/tmp/
mount -t proc proc /mnt/bt/proc
mount -o bind /dev /mnt/bt/dev/
chroot /mnt/bt/ /bin/bash
Next to edit the "lilo.conf" file, do the following in the same terminal:

nano /etc/lilo.conf
You might have to edit the following line, if you've installed to somewhere other than sda3:
Save that file and exit.

Next, in the same terminal, do:

nano /etc/fstab
Change the first line, which mounts the root filesystem, to something like:

/dev/sda3 / ext3 defaults 0 0
Replace "ext3" if you're using a different file system.

Then, again in the same terminal, just do:

lilo -v
and reboot.

That did it for me. Of course you can play around with your Lilo config file later to get booting into different OS's.