I hate to disagree but actually you don't. Your ethernet adapter which is the piece of equipment that connects to the LAN, is listed in Control Panel>Device manager under network adapters. Right click it and go to Properties then click the Details tab and scroll the HardwareIDs and note down the four characters after VEN for Vendor and the four which follow DEV for Device.
Go to http://www.pcidatabase.com and input those details in the relevant boxes and you should be led to the right manufacturer's site for the correct driver. When you click on link to the manufacturer's products, the Devices won't be in numeric order so use your browsers Find facility (usually in the Edit menu) to find the right device. You may get an executable file (.exe) which you simply double click and away it goes or a .zip which needs to be unzipped but which will probably install itself after that.