The following will allow you to use your laptop via Ethernet, note though that you can only have one pc connected at a time so you will need to unplug the Ethernet connection to the main PC first.
1. Turn off modem (unplug from mains)
2. Turn off laptop
3. Unplug USB cable from both
4. Plug Ethernet cable into both
5. Turn on modem
6. Wait for lights to settle (5 minutes, gives you time for a cuppa)
7. Turn on laptop
8. All should now be ok
Unless, of course, you meant that you want to plug your laptop into your PC and share the connection that way? If that was what you meant then I think you need to look into Internet Connection Sharing, but I'm not sure if/how well that works.
Otherwise, you could have both connected to the modem at the same time, but you would need to connect them through a networking switch or a router.
If the above isn't what you want then just clarify and I'm sure we can help sort it out