Trouble with securing network

I'm actually doing this for someone else who isn't much of a techie (and neither am I to a certain extent).

Recently, we discovered her network wasn't secure when she thought it was. When we failed to correct the problem, we suspected it could either of two things: Leopard isn't compatible with the Airport software and/or the Airport, or the Airport software and/or Airport is outdated (it's about 4 or 5 years old).

FYI, she has an airport extreme, the one that's slightly cone-shaped.

I personally tried to correct the problem by going into the airport utility, which for some reason wasn't able to detect any base station or the airport, so we couldn't view anything except this error message that no wireless devices were detected. There are two computers that are connected wirelessly to the airport.

I also tried going into the network utility and then the advanced settings where I could choose to add a new network. I tried to see if I could add a new network and add a security password that way, but once I clicked on WPA or WPA2, the textbox where I would type in the password was grayed-out so I couldn't type into it. I clicked on WEP which was the only option that would allow me to type in a password just to see if it worked. Well, it did as I saw a lock in the upper-right corner where the network icon is, however, the other computer was still able to log on without requesting a password.

Resetting the airport (by pushing that small button in the back with a paperclip) didn't work either. Also, we weren't able to find the manual or the boxes that came with it.

So, like I mentioned earlier, I'm just wondering if this might just be a case of replacing an old 4-5 year old airport. Or can it be resolved?

Thanks for your help.