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I will guess that when you boot all you see is the grub boot prompt. Grub needs a file called menu.lst which is located in the /boot/grub directory. When your external hard drive is disconnected grub halts.
You can install grub to your external drive and set the BIOS to boot from USB first. Then restore the windows bootloader. To install grub to your USB drive type in a console window.
> root (hd1,0)
> setup (hd1)
Not knowing your setup I will assume that root is the 1st partition on the drive.
It is also possible to boot windows from the grub command line.
You can reinstall the windows bootloader using the install CD and boot the recovery mode then run the fixmbr command.