Apologies if this doesn't make much sense but I'm not a Mac user. My parents have an Imac and I previously managed to set up a wireless network for them so my dad could use his windows laptop from upstairs to connect to the internet. Something has gone wrong and I'm struggling to set it up again.

The Imac is downstairs and is hard-wired to an Airport Extreme base station (the old dome-shaped one) then to the cable modem. This works perfectly.

The windows laptop is upstairs and needs to connect wirelessly. My dad bought an Airport Express to extend the range of the signal and connect the printer (also in the office upstairs) to the apple network .

My parents are not tech-savvy in any way and this is about the fourth time they have managed to mess up the wireless network. This time I am struggling to get the laptop to access the internet wirelessly. All the diagnostics and assistant menus seem to say that both machines are able to connect to the internet but the laptop isn't - it gets stuck acquiring an IP address.

Any advice or pointers as to how/where to sort this out would be much appreciated. Thanks.