Is there a way to run Vista on an External Hard Drive?

Am a Mac Newbie with some PC application baggage...

It was my understanding that "Macs" could boot from either internal and external hard drives. I thought I could (somehow) install Vista on a partition on an external hard drive and simply connect it to my Mac Book Pro when I wanted to boot from it.

My objective is to minimize (or avoid) the impact of a PC environment installed my MBP, yet still allow periodic "native" or virtual access to a PC environment as needed. And, perhaps, to use any remaining space on the external hard drive for backup storage...

Am not seeing any references to this type of setup...is any part of this possible?

If I understand the threads correctly, Boot Camp only installs Vista to the internal hard drive. And while Microsoft limits the virtual use of Vista to their Business and Ultimate editions, will virtual manager programs such as Fusion or Parallels allow me to put the PC environment on an external hard drive to plug and play as needed?

I thought native would be simpler...but maybe the route to go may be virtual management to get Windows on an external hard drive?

Had originally posted this in the Windows Compatibility Forum and a member there recommended that I post my question(s) here... Had read in the Boot Camp Installation and Setup Guide that Boot Camp can't be used to partition and install Windows on an external disk...but am hoping there may be some other approaches to meet my objectives

Thanks.