You're just having problems telling the iBook what the security "mode" is and what the password is. The Buffalo Airstation will do WEP or WPA, and then you have to know what the password is. WEP isn't secure, so you'll want to be using WPA. So, you'll use the WinXP to configure the Airstation, and to reset the password to a new value, if you've forgotten what password you used originally.
Get on the WinXP and use a browser to navigate to the Buffalo configuration webpage. It'll be something like 192.168.1.x (go in Network settings, and properties, etc, to see what's listed under router) and from the browser a dialogue will popup asking you for username and password. Username is "root", password is whatever you assigned when you first configured it. Hopefully you didn't leave it blank?
The following instructions assume that you have a 802.11g card in WinXP. If your Windows computer has an 11b card in it, then you'll have to use WEP.
Then, go to Advanced, LAN Settings will open, click on Wireless LAN Security, make sure TKIP button is clicked (not the WEP button) and then enter a WPA-PSK password. Click Apply.
Enter the password in the iBook, after specifying that it's WPA. That should do it, unless the Airstation is also configured to filter the wireless access using MAC address (= machine address, nothing to do with Apple.)