Problems connecting Apple to home network

My son has an Apple iBook using Mac OS X. He has no trouble using it at his high school connecting to their wireless network and the internet. The problem is that he can't connect to our wireless network at home. We have a Microsoft MN-700 router and another son has no trouble connecting wirelessly with his laptop (using Windows XP). If an ethernet cable is run between the router and the iBook, everything works...it's just when he tries to connect through the Airport that the problem arises. The Airport icon on the iBook will say "Connecting..." and then a message comes up that says "There was an error in connecting to the wireless network" but gives no more information about the error or how to fix it.
I have disabled WEP & WPA, turned off MAC filtering, made sure the router is set to assign the IP addresses...I have no idea what else to try. Any suggestions? I'm not sure what other information you need or if I'm even in the right place looking for help. Thanks for any help you can give!!