How can I connect Airport Express to the Router wirelessly?

We have just purchased our first iMac with Airport Express. Right now we have two networks, our existing one and the new Apple Network (Airport). The Airport Express is in the lounge connected to the home theater system so we can use AirTunes - that being the only thing connected to the Airport Express.

How can we connect our Airport Express to our existing network wirelessly? We have no reasonable way of connecting an ethernet cable from the router to the Airport Express.

Also note, there is a PC laptop which may need to be taken into account.

Nevermind - Problem Solved

When Airport was set up it had created another network, instead of joining the existing one. Settings changed and all works fine now.