First, connect with an ethernet cable to the Router, just in case you mess up the Wireless settings (changine/enabling the WEP password will cause your Wireless cards to lose the connection until they are configured to use the same password as the one the Router for the Wireless network).
Create the WEP 128 bits password/key from the Router (connect to its web interface as it is said in the manual) write the password/key down, set-up your wireless card in your PC to operate in infrastructure mode (if there is a "enable ad-hoc mode" check-box, make sure it is unchecked then) and enable WEP 128 bits on the Wireless card (Right click on My Networks and then left click on Properties.. right click on the wireless card and then left click on Properties). Now insert the password/key you previously obtained in the appropriate field.
Make sure that the channel ID is the same on both Router WiFi interface and your Wireless card on your PC.
Also, if you have time: write down the MAC address of your Wireless cards (ipconfig on Windows CMD prompt and ifconfig on your Mac G5 which should have OSX).
Now enable an option on the Router that allows only identified Mac Addresses to connect to the Wireless network and insert the two MAC addresses you just obtained.
Also make sure you know the Wireless conenction SSID (do not use the default one) because you want to disable the broadcasting of the SSID by the part of the Router.