Install WinXP to an external usb HDD

I had wanted to do this so that i could see if I could boot to WinXP on the Ext HDD from my MacBook using Grub.
However, I hadn't realized that MS has done its utmost to make this almost impossible. Note that I don't want to boot in the normal way to WinXP on the external drive so I'm not counting on the BIOS recognizing the usb drive as bootable.
In the same way, I can easily boot to whatever Linux OS on an external drive from either the Mac or Dell, neither of which support usb booting.
I found this article which describes in detail how to go about this (or so I thought).
There's another guide as well which describes in detail how to use cabarc x which is by no means obvious from the first article. I'll link to this when I find it.
In any event, I went through an awful lot of work including modifying a version of WinXP_Pro.iso to allow usb-booting and disconnecting the internal HDD in the Dell dim4550 (so that it would see the ext HDD as the only drive. However all to no avail. It seems this just isn't going to work if the computer BIOS does not support usb booting. Amazingly, of 5 computers in this house, not ONE allows usb booting. If there were just one, I could install WinXP on the external drive and do what I want.
So looks like this is a dead end.
Too bad but you can't win them all.