I assume that your setup is like this:
Cord from modem to Ethernet HUB, separate cables from Ethernet HUB to respective PCs. If your Modem is your Ethernet HUB, you can leave out the cable from Modem to HUB.
Have you checked the following:
- Cable from your current PC connecting to your other PC
- Cable from your other PC to your current PC
- Connecting either cable to another port of your Ethernet HUB?
If your current computer works on the cable of the other PC, we can rule out a hardware problem with respect to the cable and/or Ethernet HUB.
If the other PC does not work on the cable currently connected to your PC that does give access, it is something with your computer. It could be the ethernet card in the PC (hardware) or a software problem.
What windows version are you running? Have you tried rebooting with all cables connected? Have you setup an internet account on the other PC?
That is what I can think of from the top of my head...