Using Linksys Router with AirPort

My sister has a wireless network in her house. She has the desktop connected to the Linksys router via a ethernet cable and her Vaio laptop is connected wirelessly. Her new roomate has a Mac laptop and shes uses the airport extreme base station for her internet. So as of now, here is how everything is connected:
Cable Modem connected to the Linksys Router
Desktop PC connected to the Linksys Router
AirPort Extreme Base Station is connected to the linksys router
Laptop Trying to connect to linksys router wirelessly
Mac Laptop trying to connect to airport wirelessly.

Now once we plug the airport into the linksys router. Both PCs loose the internet connection and the Mac laptop still works. I tried getting rid of the linksys all together and just having everything go through the airport but I can get the desktop to connect to the internet through the airport. I know it is connected to the airport because I am able to configure the airport through the "AirPort Extreme Admin Utility for Windows" and I know that the airport is connected to the internet because it was able to update its software. I dont get it. If the desktop is connected to the AirPort, and the AirPort is connected to the Internet, why cant I get the internet on the Desktop?

 

Look at you IP information and find the MAC IP from your service provider set the mac laptop not to pull its own dns,set to local host and enter mac addy in address window,,have the mac address set manually from the apple laptop,,,this may work??? Glenn sounds like the apple laptop once connected changes the dns for the other computers....