set the Linksys up as an Access Point.
Go Start then Run in XP - Windows key plus R in Vista and Windows 7, then type in the following, including all the spaces:-
cmd /k ipconfig /all
then press Enter and from the black form that shows up, take a note of the numeric code beside the Default Gateway icon - something like 192.168.0.1.
Then login to your Linksys and change the LAN IP to your gateway's IP but ending in 10.
Disable the DHCP.
setup your wireless security. Do not use the same SSID
connect one of the four LAN ports (NOT the Internet/WAN port) to the ATT router