problem connecting Mac to internet via wireless router

So, broadband has finally arrived in my part of the world, and I'm now
online... partly.

I have a desktop PC, a notebook PC and my wife has a notebook Mac. The
internet connection is via a BT Voyager 1200 wireless ADSL modem/router,
with wireless adapters in the PC and a built-in Airport card in the Mac.

We've got the two PCs online but can't get the Mac to connect.

The Mac can connect to the router (type 192.168.1.1 in your browser and
it brings up the router configuration screen) but can't get beyond that.

Furthermore, when we try to get the Mac online, it seems to knock the
connection over and the PCs can no longer go online either. When this
happens, I have to reset the router and restart the PCs before I can get
back online.

Can anyone suggest any possible explanations/remedies?

cheers,

d.