How is the PC set up to obtain its IP address? If it's set up to acquire it manually, then you should change it to acquire the address automatically (or assign it an IP address within the range and subnet of the Extreme).
Also, if there are any software firewalls installed on your daughter's PC, you may need to add the Extreme's IP address to the list of trusted networks/zones.
Lastly, if you have WPA2 security enabled, and depending on how old the PC is and which service pack is installed (and assuming it's Windows XP), you may need to download and install the Windows WPA2 patch (don't have the link on me).