Help needed with wireless network!

We have had our Sitecom router for nearly a year, and it has never presented us with any problems. There are four people using the network (two laptops, one Mac and one desktop PC), and only one (Laptop A) has never been able to connect wirelessly.

Laptop A has had to use a wireless USB device (also Sitecom) to connect to the network since it was first set up. This laptop connects to other wireless networks with ease, and doesn't need the USB device. However, recently, it has stopped working through the USB device and has had to be connected with the ethernet cable.

Laptop B has never had a problem connecting to the wireless, until now. It is able to access the internet when the ethernet cable is plugged in, but not with the wireless. (Laptop B is running on XP)

The desktop PC connects to the internet via a USB device, and the Mac (as far as we know, but it hasn't been used recently) connects wirelessly with no problems.

Our ISP is Virgin Media, and we've never had a problem until now. Any advice would be appreciated!

Thanks in advance.

PS: Both laptops can 'see' the network when we look at Network Conenctions, but can't physically connect to it.