Linksys Cisco Router: Can't connect 2nd comp.

I bought a linksys cisco router and set it up. It runs fine until I bring another computer into my apartment. When my friend brought his computer to the apartment, he tried to connect to the wireless network, and it asked him to enter the code on the bottom of the router. When we did, it prompted him to "name the network" and set up a password. It ended up creating a whole new network and completely erasing my previously set up one. When he left, the new network stayed in place. I restarted/reinstalled my router with my own settings, but when he came over a few days later, it did the same thing. What is going on? How can we connect his computer to my wireless network without having to change the entire configuration?

Shawna