Here's my situation.

I had dual boot vista and Backtrack5-r1. After reinstallation of Vista. I got error saying:

GRUB loading.
error: no such partition
grub rescue> _

even though I have completely reformated the HDD and installed a fresh Vista.
help command is unknown, nor /?, but ls command shows:

grub rescue> ls
grub rescue>

I have googled and tried a few with no success, it keep showing me "unknown command"
I have a bootable BT on USB.
Any idea?

Thank you.