Computer pros, can you help a computer rookie? I can't get my mom's

[peanutbuterjellytime]

laptop to connect to our new WiFi? It seems as if only my Windows 7 machine can connect to our new WiFi, but *only* with the use of its *built-in* adapter. I tried connecting to the WiFi using the same computer, but with a Wireless N adapter, but I couldn't connect. It shows that it's connecting, but it never does. I tried connecting my PSP to the WiFi, but I could tell already that it wouldn't connect. It kept going back and forth with "Connecting. Please wait..." and "Exchanging key information. Please wait...". I let it try to connect before when it was doing this, but it didn't. My mom's Windows XP laptop won't connect either. I entered the correct passcode on each device every time, but neither of them [except my computer, of course] would connect. With all that said, can you help resolve my newest computer-related issue? Thank you.
This new WiFi is from AT&T, and I've already bugged them enough.