[Chris]

There has to be an ethernet card in both machines, otherwise there's nothing for him to plug the ethernet cable into. It won't fit into any of the other holes you will find round the back of a PC or laptop. It may be that the ethernet is built into the motherboard of the PC, in which case you could technically say there is no 'ethernet card', but that's just semantics.

If you want to know will it work for you, then technically yes, if you connect a PC to the modem with USB, and you have ethernet ports on both PCs you want to use, then you can enable connection sharing on the PC connected to the modem, and then wire it to the second PC with ethernet, and it will work.

However it's better to buy a cheap router to do the job, so you don't have to have the main PC switched on just to use internet on the second one.