To keep the existing modem you will need find a router with a USB input -
You will probably be better off replacing the existing modem with a modem router.
You will then be able to connect to your desktop with a wire to the LAN connection.
Each laptop must have a PC Card slot into which you insert the Wireless Card.
I normally buy Belkin kit but the reality is they are all much of a muchness.
Just to give you an idea click here click here