The typical setup is that the Ubuntu installer put GRUB on the MBR (not a partition), and put the GRUB config file in /boot/grub (In your case on the "/" partition).
To add another OS:
Run the installer, select manual partitioning, create a new partition and install.
When you get to the bootloader, you have two choices:
1. Let it install the bootloader. It will pick up the Windows install and possibly even the Ubuntu. If not, then add an entry for Ubuntu in the (new) /boot/grub/menu.lst.
2. Do not install a bootloader as part of the new install. When installation is complete, reboot back to Ubuntu, and make an entry in Ubuntu's /boot/grub/menu.lst.
If the installation is as I am guessing, then removing the XP partition would not affect your ability to boot into Ubuntu.
I'll leave booting from external devices to someone more knowledgable......