Let Windows redetect drivers
If you have not recently reinstalled Microsoft Windows, it's possible that your network drivers are simply corrupt. Open device manager and remove all devices under "Network Adapters" and any "Other devices". Once removed, reboot the computer and let Microsoft Windows re-detect the network drivers.
Motherboard chipset drivers
If your computer came with a Motherboard CD, install the chipset drivers from the CD and the network drivers if also available on the CD.