HELP! WIRELESS ROUTER NOT WORKING!

Hello,
My mac desktop has always had problems with the router and loses connection frequently on and off.
But my laptop and my brother's have never had any problems, until now.

Even when there is 'excellent' connection and I can see my router on the wireless list but it tries to connect then eventually says 'Cannot connect' and on the mac 'Connection timed out'. The Windows Vista laptops can access the internet via a cable, and this is how I am writing this. But the mac cannot. I have called up the helpline and they were basically no help at all - they just told me to do everything that I have already done restart the router, set up a new connection, try restarting, try setting up a new connection. None of these work, and the network diagnostics on any machine say they cannot fix the problem.

Can someone please helpl me as there is only one ethernet cable and this was the reason why we have wireless in the first place - plus when we go back to school my Mum won't have a computer that works with a cable.

HELP
P.S I am sorry if this topic has been brought up before I have spent the last 6 hours trying to fix it with no prevail
Thanks