Help with Airport Extreme Base Station- Can't Connect with >1 computer

[descartes]

I have an apple airport extreme base station. Not the new "N" version but the one prior. I also have comcast cable internet. I set up the airport and I can connect with my imac no problem. However, my other two computers (pc laptops) and apple TV can establish network connections with the base station but cannot connect to the internet, ie, computer shows connection but the internet does not work. any ideas? i have tried setting various settings of security but nothing works. Meanwhile, my imac (intel) connects no problem. Thanks.