HI,
I have successfully installed XP to 512MB USB device. Used SP2 as it has
USB2 native support for speed. Used XPLITE to remove components I do not
need (games, IE, plus, plus) and did not install any HW spcific drivers (VGA
etc.). There was even a bit of room left to install an antivirus prog. and
some small utils.
Changed the boot order in BIOS, and the PC's I have tried it on boot from
the USB device.
I have not tried it with 2000 or NT, but beleive NT will be difficult
because I have experienced limited USB support with with NT4.