First, you need to pick one of those computers as a "server". Connect it to the Internet using the usual way. Next, open Control Panel>Network and Internet Connections>Network Connections. Open the Wireless Connection Properties. Change tab to Wireless Networks then click Add... button. Under the Association tab, input your Network Name as you like. Clear the The key is provided for me automatically check box and select the This is a computer-to-computer (ad hoc) network check box. Fill in the Network Key along with the confirmation with a 13-digit password and then press OK. Press OK once again to close the dialog box. Now you have finished with the first computer.
For the other computer, check for available wireless network. Usually a notification balloon will pop up and you can just simply click on it or just go to the Network Connections, right click on the Wireless Network Connection and click on View Available Wireless Networks. Select your network, and click connect button on the lower right corner. Input the Network Key and press connect once again.
If the first computer is using a PPPoE connection to gain access to the Internet don't forget to allow other computers to use it. You can do it by accessing the Network Connections, then open the properties of the connection. Open Advanced tab, check the Allow other network users to connect through this computer's Internet connection and set the Home networking connection to Wireless Network Connection.
In my experience, this network is not very stable. Not really sure what is the cause, but still good enough for a temporary solution.