|It would be useful if we knew what your system spec was then we would be able to better advise you.|
Is this a new problem? Were you able to connect to the net before?
Normally if your system came with a driver disc then the NIC driver would be on that disc. Maybe you have not installed your NIC driver. Is this possible?
Is the NIC part of the motherboard? Or is it on an add on card?
As you are able to go onto the net using another PC, you could download SIW - System Information for Windows from the net using the link giveen below. This will allow SIW to interrogate your system and tell you what your hardware andd maybe NIC (thernet interface) capability and information is. Then you can download the appropriate driver.
☺ When everything else fails, read the instructions.