We have a Wireless network

Question

We have a Wireless network set up at home using Airport on a Mac with the Mac acting as the hub for internet access. Most of the computers in the house are Macs and work on the network quite cheerfully as does my work PC with XP installed. Our problem is we have just added 2 new laptops to the mix, 1 with XP & 1 with Vista. We ccan't get either to connect to our network. They can detect it but won't connect. We've tried every possible suggestion we can find on the net - none have worked so far, any ideas?