M/Lion and Wi-Fi networks

Happy New year to all.

Not sure if this the correct place to post this, but here goes.

I have an "old" Wi-Fi network which still exists which was set up some 6 years ago when all I had was a Macbook Pro and an Airport Express 802.11g. and the AE being connected to my wired ADSL modem

Since then I have added an iMac QC i7 27" (late 2009 model) and more recently another Airport Express 802.11n.

Now the problem I have is that whilst Wi-Fi on my iMac is not switched on, I can still sync my iPad 3, and iPhone 4s and MBP from the iMac to these devices. Obviously the system is syncing via the Back-door so to speak ( thru my old AE and modem).

I have now received a Philps HDMI Media Player with Wi-Fi which will not allow me to connect to my network system as I am unable to input my

password etc.(won't accept) it - connection fails.

I'm now thinking that perhaps it would be better to completely un-install my wi-fi system and start afresh using my iMac as the master with Wi-Fi turned on.

How and what is the best way to remove all traces of existing sytem and then how do I set-up a new network?

If anyone can explain in laymans terms so much the better.

Alan

27-12-12