If you have an XP CD you can make a usb bootable disk quite quickly.
"I am running XP on a USB drive".
The basic principal as i understand it is...
USB devices are not seen by Windows set up as the drivers are not pre loaded like IDE and Disk (CD/DVD) Drivers.
USBoot enables pre-loading of USB Drivers.
Some small changes are also required to some files on the installation CD.
(extract, modify, re-integrate, burn new CD).
Why not take a look at the links provided ?
can't be bothered ? "no not really"