Is it possible to connect a laptop to a cable modem through a desktop CPU to avoid running another separate network cable from the modem itself ?
I want to be able to go online and use the laptop as a second monitor without running another cable.Any info you could give me will be greatly appreciated.Thank you.

Sounds like you are wanting to run a wireless connection. You'll need a wireless (internal/external) interface.

Only if the other PC has 2 Network cards. Even then I wouldn't recommend it. It would be better to get a wireless router.

I already tried a wireless router but for some reason it ran really slow, only about 2kbps.