how do I temporarily connect my macbook to someone else's cable modem/internet connection using their ethernet cable?

I'm visiting family that have a cable modem (through cablevision) but no wireless. They have one (1) HP PC hooked up via ethernet cable. I would like to temporarily hook my macbook via ethernet to their internet connection. Is this possible? If so, how?

I'm currently running mac os x 10.5.8

Thanking anyone in advance that can spare their time, knowledge and patience.