I'm trying to setup a network with three computers. The modem, router, and two of the computers are in the basement and they are all problem free. The cable connecting the third computer runs up the wall and into the attic and then down through a wall in the room where the third computer is. When everything is connected, the network and third computer do not detect each other. I've checked the wiring over a couple times and I'm positive there is no cross pairing of any wires. I've also connected it directly to the router and there is no prblem there. Why won't the computer connect to the network? It's driving me up the wall.

The fastest way to be sure:
Move the 3rd computer down (from upstairs) and replace it with one of the two computers down stairs and see if it will connect then.  If it does, then there's no doubt a cabling issue going upstairs.