Upgrade Leopard to SL.... lost wifi

After upgrading a mac mini from leopard (worked fine wirelessly) to snow leopard... I find that i cannot link into the router wirelessly. It can see the connections available (mine and neighbours connections) but when trying to connect it ends with a "connection timeout".

Non of the 'fixes' I have found work at all....

system pref plist deletion and reboot

altering mtu settings in network and on router

manual ipv4

adding in dns

I have a macbookpro connecting in to wireless and it is working. It is running leopard. When i upgraded it to snowleopard it would not work either, so i set it back in timemachine to a working state.

However i need Snow Leopard to work properly.