In your BIOS (if supported) set your boot order to include USB storage or HDDs (BIOSes will list variations but you'll figure it out once there).
If the XP install is intact, you may be able to boot from the device. It will, however, be much slower than if connected via IDE or SATA.
Also, you should also have an option to manually select the boot device when the system is booting up, simply select the USB drive.
Good luck!