You'll find that the instructions you linked to are the "tip of the iceberg." Modifying the install CD is extremely "non-trivial." As released, Windows does not support booting from an external drive because during the boot process the OS re-enumerates all the ports, losing the connection to the boot source in the process. The hack in the link from MacRumors requires some pretty strong work and can result in unpredictable end products-you may have to step back and start over, not just wiping the hard drive and starting the install over, but rebuilding your modified CD-over and over.
This process is not for the faint of heart. Good luck.