Connecting Wireless Router to Wired Router

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I have a huge task at hand. A computer running XP Home. Currently it is connected with a wired router to high speed ADSL. The task is to set up a wireless router, running out from the wired router. So far, I know that I can simply clone the mac address from the wired router to the wireless router so to eliminate having to call the ISP to register yet another mac address. Question: Is this the WAN or LAN mac address I will be cloning? There is nothing else on the network but the one computer. They just want it to be wireless. Second question is setting up a secure connection. I understand that WEP is the way to go? How to I make an encrypted pass phrase? Both routers are Linksys. Any and all explanations would be very much appreciated.