owp sorry I was think of this -

http://blog.tmcnet.com/blog/tom-keating/gadgets/boot-windows-xp-via-usb-key.asp

"All that’s needed is a bootable USB Flash drive with at least 256 MB of storage capacity and a Windows Setup CD. Using the program Bart PE Builder (Freeware), you can install Windows XP on the flash drive, along with other software as needed (and as available space permits)."
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Here's What You Need to Use Windows XP Professional

•PC with 300 megahertz or higher processor clock speed recommended; 233 MHz minimum required (single or dual processor system);* Intel Pentium/Celeron family, or AMD K6/Athlon/Duron family, or compatible processor recommended
•128 megabytes (MB) of RAM or higher recommended (64 MB minimum supported; may limit performance and some features)
•1.5 gigabytes (GB) of available hard disk space*

1.5GB is the space needed to install all features and have a
copy of the installation files. I've got bootable versions
(from MS for OEM's) that take up considerably less and there
are other stripped down versions on the interweb that can fit
on a 256 MB flash drive. I think I also saw a version less
then 128MB. I'd have to search again for it. All depends on
what you want to call Windows XP.

Regards.

e