I have never booted off an external hdd. Ihave researched it and come to the conclusion that it is very difficult, and time consuming.
Editing your grub is fairly simple. If you are using Ubuntu, I highly recommend using the Ubuntu forums, they have a great community of helpers over there! To edit grub:
1. Open the terminal
2. type - sudo gedit /boot/grub/menu.lst
3. You will need to put your password in, and it will open up a text editor with your grub menu list
4. To set windows to default, you will need to find the line
5. Change that number to the number of your windows in the list.
Note: every entry is counted as a number, even the memtest and recoverymode.
6. Reboot and see where your default falls, you may have to edit it again and add another number until you figure it out.
Also if you have a kernel upgrade, you will have to edit your default # again as that will add another boot option to the menu.lst, and not compensate with the default number.
Hope that helps.