Yes you can James, it's pretty simple. In XP, once the cell connection has been created, disconnect, go into Network Connections and find the icon for the cellular connection. Right click on it and click properties, click on the farthest tab on the right (Sharing) and check the box that says "Allow other computers to access the internet through this computer".
Once you reconnect to your cellular connection, you can create an Ad-Hoc wifi connection and anybody who connects to that wireless network will have internet connectivity through your air card.
In Win 7 the process is the same, but you'll have to open your Network & Sharing Center from Control Panel, on the left side click on Change Adapter Settings. Find your cellular connection and the steps from there are the same.
