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 word "gateway" -- usually something like 192.168.1.1. Type exit to close the black form.
connect only a computer to the router you are trying to convert to a AP and log in
set up the wireless security
disable DHCP and change the LAN IP to match your Gateway's IP but ending in .253
now connect one of the four LAN port to the main router via network cable.