The SAGEM [email protected] 800-840 is infact two models, the 800 and the 840. Neither have any Wireless capability so you cannot connect to them with your laptop.
You would probably be best buying a Wireless capable ADSL Router and replacing your Sagem. You could then use the WiFi on your laptop to connect to the wireless on your Router whilst your desktop would continue to connect through a LAN cable (I suspect you currently connect through your USB).
Lots of Wireless Routers are available click here to see some alternatives.