I installed BackTrack 5 r1 and I can't select boot option between Windows and BackTrack on start up.
How can I fix this, or how can I install grub 2?
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How to on Dualboot with Windows & BTLinux is explained here : http://www.backtrack-linux.org/tutorials/dual-boot-install/
However, I am assuming that you have had a problem with the grub installation itself and trying to find ways to get back to the dual os boot prompt on grub.
When you boot the machine, does the grub menu/splashscreen appear asking you for the OS boot prompt?
If you are not getting the grub prompt and being directly taken to the BTLinux boot sequence, then please follow the steps to get grub installed first. You need to install grub onto MBR of the harddisk (not the partition where the Windows resides)
Once you login as root under BackTrack Linux, check whether you have grub installed, usually found under /sbin/grub and /sbin/grub-install
Its assumed that /dev/hda is your harddisk on which you want to install the grub on the MBR. If its some other notation like /dev/sda or /dev/sdb, then follow that notation. In the above snippet, with grub-install, we have installed grub's first boot loader onto MBR; with grub shell, the root disk is setup. The menu configuration files will only have Linux entry, now update that with Windows entries.
Make the changes as below: