Adding laptops to a wireless network

Can anyone help with this problem please?
We have 2 friends staying with us for a few months, they both have laptops (running XP) with wireless network adapters and we want to give them access to our network. Their laptops can see a neighbour's wireless network, but not ours, which suggests their wireless adapters are working. Another laptop can see both the neighbour's network and ours, so the wireless part of our network is working. The 2 friends' laptops have the encryption key for our network and the SSID is being broadcast. Any ideas please? Is it likely to make any difference that the friends are from the Netherlands and brought the laptops with them?

Thanks

brand + model of each bit of offending gear.

your not using MAC filter lists on the AP and forgotten about it (i've done that :D) using SPA/WEP which the laptops don't support?

Thanks for the suggestion, Animus. Tried them but still no joy.
The modem/router is a BT Voyager2100. Laptop that works is a Mesh, with a 802.11g miniPCI adapter.
The laptop that doesn't work is a FujitsuSiemens Amilo with a ralink RT 2500 wireless LANcard.
All the settings we can see through Windows are the same for both wireless adapters, both have the name of the modem/router and the same encryption key. Also tried connecting with the encryption switched off, but that didn't make any difference.
Totally lost as what to do.

Are you sure both are the same format, IE if a 11g card works - the other cards may need to be g as well rather than a or b.

Thanks Unreal, but tried that too. The router should auto configure for b or g, but we tried setting it to just b, and then for just g, but no luck. Thinking of trying different wireless cards now, but any other suggestion will be welcome.