You should have no trouble doing this at all.
You have been running Backtrack, based on Ubuntu.
Ubuntu is based on Debian.
Kali is based on Debian.
As far as grub goes there should be no problems at all. If there are there are a couple things you can do to make sure everything works.
Backup to USB Stick or optical disk your /boot/grub/grub.cfg file. This is where your menu entry is for your MS install. This can be used to get you an entry for that install in your new Kali install if every thing else fails.
When you install, assuming they use the Debian installer, it should list for you the other OS's on your HDD. See if MS is listed there.
If not go ahead with installing grub to the MBR.
Check the grub screen menu to see if MS is in there. If it is it will work. If not don't panic, simply boot to Kali.
When booted to Kali open a terminal and, as root, run;
This should fix the problem right there.
If it should for some reason not do that copy your backed up MS menu entry and put it in your Kali custom menu entry script, again as root;
Save the file and then rerun "update-grub".
It is very rare that grub, when installed by any distros installer, fails to pick up a MS install.