Hi Thupten,
Welcome to Microsoft Community Forum.
I can understand that you have formatted you hard drive and reinstall windows now you are not able to install the network driver. Not to worry we will guide to get the issue resolved.
I suggest you to follow the below mentioned methods and check if it resolves the issue.
Method 1
Run the Network adapter troubleshooter
http://windows.microsoft.com/en-us/windows/fix-network-adapter-problems#1TC=windows-7
If the issue persists, follow method 2.
Method 2
Uninstall and reinstall the network adapter
- Press Windows key + R
- Type ‘devmgmt.msc’ in the run windows and click ok
- Click on network adapter
- Right click on network adapter and click uninstall
- To reinstall the network adapter, download the network adapter from the below mentioned link and once you are done with the installation restart the computer
Hope this helps. Write to us for further assistance.