How to remove any kind of Airport Express settings on computer

My boss gave me an Airport Express 802.11n Wi-Fi to "extend" his current network which runs on a Belkin router. He has another router (linksys) that works in the office area, but wanted to eliminate the need for this router and run everything off of the belkin; therefore having everything on the same network.

Now I was simply getting acquainted with the device and after I plugged in the ethernet cable for the linksys, it has totally screwed up my wireless settings on my laptop. I'd like to go back to just being able to connect to the linksys without having to have the AX plugged in. But as it stands now, any time I unplug the AX, I then cannot connect to the linksys at all. It shows up under my available networks, but anytime I try to connect to it, it times out. This cuts me off from the printer/scanner/fax machine.

Is there any way that I can just remove all this crap from my wireless settings and start over? I've tried resetting the AX, restarting my computer, and whatnot. None of that has worked.

Also, I was told by a friend of mine that the AX is only to be used with an AirPort Express Extreme. Is this true for all uses of an AX or just the extension of an existing network. Any advice on how we could efficiently distribute the Belkin router's signal to the rest of the house/office would be greatly appreciated!