Most important step in port forwarding is the very first step:

1. Setting up a Static ip for XP (click on link)

2. Port Forwarding instructions for the Motorola SBG 900 (click on link). (Port forward NAT firewall rules are based on IP address which is 'one' reason you need to set up static ip first. Other reason is you do not want the ip address to change else the NAT firewall rule will not work.)

Choose either port 6346 in all port settings, else choose a port between 49152 - 65535 such as 64049. This range of ports are safest from ISP (Internet Service Provider) interference.

Take it slowly, step by step, once you have done it once you will realise it was not as hard as it first looked.

Oh .. not sure about Motorola but some devices you need to save settings and reboot the device. Else, it is at least wise to reboot your computer after doing this.