It will be extremely complex to set up a network while keeping the USB modem as your broadband connection. You will need to make the desktop PC into a router for the laptop to access the net. I wouldn't be brave enough to go that route - I don't know how to do it, although it is theoretically possible.
Your BT BROADBAND: BT Voyager 105 USB ADSL MODEM is designed to work through a USB port on your computer, and to feed that computer only.
If you want to connect more than one computer, then you need a modem and a router. The configuration I suggested above (or one like it) will do that. My daughter has a BT Broadband connection, and was provided with the same Voyager 105 USB modem; that modem is still in its box (unopened). She has a D-Link ADSL modem router and a D-Link wireless access point and it all works flawlessly.