Here's what you must do :
# wget http://bt.gxrg.org/other/tools/script/shell/bt4-5-manual-install/install-bt5-r1.sh
# sh install-bt5-r1.sh
Answer all question from the script about which hardisk partition that will be used to install bactrack.
After that when there's a yes/no question, answer all with yes then wait until the installing process finished.
Then we must install the grub.
Exit from the chroot, (done by pressing CTRL+D), then type this command
# grub-setup -d /tmp/bt/boot/grup /dev/sda
"sda" here means first partition of the harddisk (master boot record)
After that, reboot your system then update the grub, to install grub menu and dual OS.
# update-grub
There you go..
Your backtrack ready to rock.
