Dual Booting Ubuntu on an external Hard Drive

Can anyone recommend a very easy way to install Ubuntu on a USB connected external HD, for dual booting purposes? I've read about some problems with something called grub and the MBR and since I don't have any install disks (gateway FTL) for XP, I would REALLY like to avoid accidently formatting my C: drive (or even needing to do it on purpose).

How about installing Ubuntu on the external drive and setting your BIOS to boot from that first? Then you can go ahead and add Windows to your GRUB configuration.

Acronis Disk Director. Get a crack from tah internet.
nuff said