This is why we advocate to people just like you, to not use Backtrack! A common example of messing with things in which you have no knowledge of.
So it appears as though you have installed BT to your HDD, and you have set grub up in your USB stick. So that means in oder to boot you need the usb stick to be inserted first.
In order to fix it, well that's probably a matter of you getting your windows media disk out and using the restore or repair option.
See also Grub 1.5, Error 21 in google for more info.
I would highly suggest that if you want to learn linux at least start with something more noob friendly, such as ubuntu.
See also the links below.