Trying to use AD HOC....in need of help.

So I have a HP desktop pc and a Compac laptop. The problem i'm having is that my pc is connect to the internet through a Sierra Wireless 881 UsbConnect Aircard, i'm trying to connect my laptop to internet through the pc since i only have one card. everytime i set up an AD HOC, i meet be connected with both computers at once for about 30 minutes. After the network that the laptop is connected to the computer on, keeps going to "local only". Can someone help me out? Am i going about it all wrong or am i missing something. Please help...... Thank You.