First you need to shrink the windows partition to make room for the linux ones.
Then you need to create 3 partitions.
The boot partition /boot, the root partition /, and the swap partition /swap.
The boot only needs to be small (about 10Mb or so) the swap should be 1 and a half the amount of ram you have and the rest for root. Remember to format the partitions correctly after creating them (i.e makeswap, etc)
When you've created the partitions and formatted them, then just boot from the backtrack 3 cd (At this point it's handy to issue the swapon command to activate the swap partition) and use the installer supplied on the backtrack cd to install to the correct partitions.
To set the lilo loader to also boot windows edit the /etc/lilo.conf and add the lines
other = /dev/hda1
label = Windows
(change hda1 to reflect where your windows is installed.)
hd denotes ide drive
sd denotes a sata drive
a,b,c denotes first,second,third drive of that type
1,2,3 denotes first,second,third partition.
So for first partition on the second sata drive it would be