"Invalid System Disk" error when booting BartPE

Hi,

I am using the instructions from this guide:
http://www.tomshardware.com/2005/09/...n_your_pocket/
to install Windows PE (slimmed down version of XP SP2) on a USB stick.
I successfully completed the instructions and my BIOS settings are
good, because the BIOS recognizes the USB stick, however when I try to
boot from the USB stick, I get a "Invalid system disk. Press any key
to continue..." error. I also tried using a RAM disk, however, that
didn't make a difference. Please HELP. Here are the neccessary system
specs:

Motherboard: ASUS P5ND2-SLi Intel Edition
BIOS: Phoenix Award BIOS
USB Stick: 512MB Kingston Data Traveller
BartPE version: 3.1.10a

Thanx