Troubleshoot your system per the TSRM. Undoubtly, you have something not working correctly, when our heaters work, they work extremely well. Is your system blowing warm air at all? If not then you have an open or a short in the system somewhere. If it is but not hot, then my guess would be the water valve or the vacuum switch that turns it on and off. Also, be sure your coolant is full and completely burbed of air. An air bubble in the heater core will greatly reduce how well it heats.
If the system is full and the engine is coming up to temp, you can bypass the heater control valve and hook it up to run full heat. You can rig some manual valve if you want some control.
If the windows continue to fog, your heater core may be leaking.