If your normal internet connection sharing is not working, you can bridge the ethernet connection and the wireless connection on the computer with internet access. This will allow other computers connecting to the ad-hoc network to access the internet through this computer.
To do this follow below instructions:
1. Right click the network icon the two computers on system tray
2. Go to Network and Sharing Center
3. Click on Manage network connections
4. Select the Local Area Connection (the connection you are using to connect to the internet)
5. Select the Wireless Network Connection (hold CTRL button and left click)
6. Right click on the selected connection
7. Click on Bridge Connections
