that ,,Windows Vista/Longhorn,, has nothing to do with the problem i think. You could even change it if you want by editing menu.lst of /grub/boot folder.Haven't installed BT4 on my HDD yet and also it's been ages since i edited my dual boot menu of Win7/Ubuntu, so i can't tell you the exact path but it could look somehow like this: /grub/boot/menu.lst obviously if you want to edit this then you have to run this from command line: nano /grub/boot/menu.lst then scroll down to where you see ,,Windows Vista Longhorn,, and change it for ,,Windows Xp Proffesional,, lets say.
see here: How to Edit the Grub Menu in Ubuntu | eHow.com (don't have to use ,,sudo,, if you are root and instead of ,,gedit,, use that ,,nano,,.)
I think the problem is that when you click on ,,Windows Vista/Longhorn,, it tells Grub where to look for windows system files to start boot Windows. Likely that path is wrong or files are corrupted, see when editing that name of Windows , i think you could see it there as well. (i mean the path, not corrupted files.)
Also i am not giving you a guaranty i am right but it could be one of the options.
Maybe a reinstall of Grub could help.
....assuming that BackTrack still uses GRUB not GRUB2 ...