I see you asking a question, but then saying you're not going to attempt the solution.....???
Assuming that you can configure your BIOS to boot from an external drive, then:
1. Install GRUB to the external drive. Simply boot into Ubuntu and open a terminal. Then:
## First, I am assuming that your external drive is seen as #2. To check, do "fdisk -l"
sudo grub ## starts the grub shell
root (hd1,0) ##this assumes that your /boot directory is on the first partition (grub counts from 0)
2. Use your Windows disk to re-install the Windows bootloader. Boot from the CD, enter recovery mode, and enter "fixmbr".
You can now switch between OSes using the BIOS settings