Can I Set Up Two Modems?

[john]

Here is the situation, I have a computer downstairs feeding off a wireless signal from a WAP and a modem. Those are both upstairs.

What I would like to do is connect my downstairs computer using an ethernet cable.

Is it possible? And if so what do I need?

Would I need to buy another modem and hook it up downstairs? Is it possible to set up two modems?
So basically I cannot run two modems? Okay. Looks like my best option is to run a booster aerial. Since I cannot run an ethernet cord through the house and do not want to buy another account or phone line.

Just for curiosity sake, what would happen if I hooked up another modem downstairs? Would it not work? Would it cause problems or get me in trouble with my service provider?