Instructions for creating an ad-hoc network vary between Windows XP, 7 and 8. We highly recommend using Connectify Hotspot software to create and manage ad-hoc networks. The free version worked great last time we checked.
Once you've created the ad-hoc network on your Windows machine, connect to this network with your tablet:
Note: Starting with iOS 6, it seems that the AirPort symbol no longer appears when the iPad is connected to an ad-hoc network, even if the connection is properly established. You can check if the iPad is connected: