You should really do a search on forum, cause that answer is covered in a lot of topics...
Try this on the boot select screen:
If this bug appear when you are using Backtrack 5 R3 Live USB you can press "Tab" button on the boot option menu and this code "i915.modeset=1" after "vga = 791 splash text" and the press [enter]
Else you can try this:
You can edit Grub configuration on /boot/grub/grub.cfg and find the Backtrack boot menu and add this code "i915.modeset=1" (same as before) after "vga = 791 splash text".
You have more help on threads over the forum