Sam,
In order to use the linksys router you will need to connect the PC & laptop either by ethernet or wirelessly.
Apologies if this is too basic, but
this picture should help you identify whether the PC has a ethernet port. If not, as Stu said you will need to get a card for the PC,
such as this. I would be surprised if the laptop doesn't already have an ethernet port. Along with the network card you will also need RJ45 cables. The router comes with one cable, which can be used to connect the cable modem to the 'WAN' port on the router. You will then need two cables to connect the PC & laptop (unless the laptop is going to use wireless!) - you can get them from most e-tailers or PCWorld.
Hth