I have a G4 and a windows (W) laptop - and have the Airport Extreme Base Station (AP) working well for both products. When setting the AP, make sure you have chosen the button in the Network choices that "allow for this computer to create networks."
Also, in the AP Utility, under the Network tab, be sure to choose the button that allows the AP to distribute IP addresses, then chose the format you wish to do this by (either sharing a single IP via DHCP and NAT, which is how I have mine)
or the other option.
then, in your W machine, be sure to have the software utility you are using set appropriately, be sure to include any WEP numbers you need to allow you to get into an encrypted network. This is a long number that can be found by opening the AP base station utility and then hitting the "Password" tab at the top.
It took me a couple of tries to get this configured, but you'll get there.
PS I got the same biz from Apple when I tried to tell them i was working between an AP and my Apple and a Windoze machine...