When installing backtrack it allows you to edit the partitions and re size if needed. Use this program, as resizing via the windows "disk management" cannot, and will not make partitions bootable. While installing, select the partition you wish to install on and hit delete. The click create partition (I think that's the button) and at the top you should be able to set it as primary or logical, select primary and format it. Then it should show a check mark on the main partition screen under the box labeled as "format" (Again, I think that's the box name). You should be able to continue and it should auto install grub dual boot manager. On your next reboot you should be able to select the backtrack boot or "Vista/Longhorn" boot.
Sorry if that is not what you are looking for.