You'll come into issues running it on more than 1 PC unless all PC's you intend to run it on have the same motherboard, CPU, chipset, graphics card etc.
If you want a portable OS to run on multiple PCs your best bet is linux, it's not so driver reliant.
generic drivers.
Like in bartpe disks.
to be honest i dont know how exactly those work but i do know ive been able to run bartpe on every pc ive come accross so far.
take the drivers from that and replace the ones yo have with them and it might work.
EDIT:never mind use this Install Windows XP From USB Full Software Downloads - Download For All
That link is for the opposite. He wants to boot to the desktop from the USB drive.
The installation will fail as soon as the setup's mini-kernel initializes the USB drivers. You need to make extensive modifications to the setup files to get it to work.