So you currently have two computers connected.
These will be still on the network at xmas as well as two new laptops, total of four computers.
Is this correct?
What type of cable modem are you using at the moment? Is it USB or Ethernet?
If its Ethernet than the simplest way to enable a wireless service is to get a wireless router, something a linksys click here
This just connects to the Ethernet cable modem then is just a matter of configuring the router.