Go into Control Panel>System>Hardware>Device Manager, find the ethernet controller which may have a yellow mark beside it, then right click that and go to Properties.
Click the Details tab, then scroll to Hardware IDs 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 will see the list of all the right manufacturer's products but sadly, they aren't in order. Use Internet Explorers Edit menu then Find to type in the DEVice ID and the search will find the correct entry. Follow the link to the site for the correct driver and download it.