Format the USB flash drive as FAT32, copy over the Windows 98 boot files, which can be found via Google search. Then copy the contents of the XP install disc.
May need a borrowed Windows machine to make the flash drive bootable (to a DOS prompt).
Next: move the flash drive to the target machine's USB port and boot to a DOS prompt. Navigate to the "I386" folder, and run "WINNT.EXE" to begin the installation.