Resetting a wi-fi network back to 'out of the box'

Hi

Sorry to ask what may be a very basic question, but how do you set a wireless router - plus your laptop's wi-fi settings - back to how they were when they came out of the box? In other words, I want the wireless routers and laptop I have to forget they know each other so that I can set up my wi-fi network again as though I'm setting it up for the first time.

The reason is a little involved but in essence I had a wi-fi network set up and running from an Airport Express and this was working fine. Over the holidays I then bought an Airport Extreme as I wanted something a little better for my base unit and I then wanted to use the Express as an extender. How I wished now I hadn't fiddled with something that wasn't broke, as the addition of the Extreme to the network was not as straightforward as I would have expected and my whole wi-fi network seemed to go wrong as did my internet connection. I got someone to configure both routers properly (figuring I may have messed up a setting when I was trying to set it up) but still no luck - it would work for a bit and then my internet would just go down. Using Airport utility showed that the routers and internet would be running for a few secs and then would stop and I'd lose connection.

I work and live on a university campus so the internet to the house is supplied through an ethernet socket in the wall, much like you'd have in an office. Through contacting the IT helpdesk they were able to tell me (though partly because of techno speak and a bit of a language barrier as English isn't the first language here) I far from understood. What I could gather was something like the SSIDs of the two routers needed to be different as they were 'flapping' and this was then causing the network to automatically shut off to the house??? Unfortunately they only deal with providing the internet service to the house and can't help with any wi-fi issues.

I am pursuing other means of help (we're a science and tech research university after all!) but one thing I was thinking was whether I could just reset the routers and my laptop's wi-fi 'memory' of the network and start again as though I was setting it up for the first time. I don't want to plug the router in and it says something like 'this used to be part of XX network'

Anyone know how?

Many thanks