I assume you have installed the Livebox and can connect to the Internet. Also that the other computer is a laptop with card inserted.
On the laptop right click on the network icon at bottom right and choose show available wireless networks. In the window that appears right click on the network icon and choose properties. Hopefully you will receive a message that there is a problem with the network. Click Change advanced settings on the left and choose the wireless network tab. Put a tick in the box to let Windows configure the network and ok out. Don't touch anyhting until its finished.
Also make sure that both pc's have the same Workgroup name. Right click My computer/ properties. Choose the Computer name tab.