Setting up an Ad Hoc wireless network and enabling Internet Connection Sharing are separate procedures. It is quite possible to have an Ad Hoc network between two computers for transferring files without either being connected to the internet.
In the Asus PC, go into the Network Connections folder, right click the Local Area Connection which, presumably, is the PC's internet connection and select Properties. Click the 'Sharing' tab and tick the box "Allow other network users to connect through this computer's Internet connection". This will give the Wireless Network Connection the IP address 192.168.0.1 and Subnet Mask 255.255.255.0 - the "client" computer gets its IP address automatically by DHCP from the Asus PC. Now set up your Ad Hoc wireless network.