Have you tried accessing the configuration manually? `Configure Livebox` shortcut on your desktop.
Not very familiar with the livebox software but look for anything mentioning security and DHCP. Check there are no restrictions on the number of IP addresses the
box will allow/allocate (starting 192.168) and make sure the MAC address of the new device has been added to any access control lists. You can find out the MAC address of the other PC by going to Start Menu/Accessories/Command Prompt and typing
Assuming XP, for 98 the command is winipcfg