well since you are a windos guru take the hard disk out of that box and temp install it in a windos box as secondary disk (slave) or primary disk (master) on the second ide controller. if you have done this read here
http://doc.m0n0.ch/quickstartpc/ how to continue. on the dowlload page
http://m0n0.ch/wall/downloads.php and download physdiskwrite 0.5.1 and generic-pc-1.231.img. use physdiskwrite to write the image on you firewall disk. that's it.
basically it's no more than download the image and write it on the hard disk in your windos box and build it back in your firewall box. BECAREFULL WHEN WRITING THAT YOU CHOOSE THE CORRECT DISK otherwise you will loose your windos install...
one more point, make sure that you correctly place the jumper on your hard disk when moving it around and make it master or slave.