2001 Pontiac Montana overheating and blowing cold air

Try warming the Vehicle, then carefully feel the heater hoses, are they both hot? If one is cooler than the other, You need to flush the core, or do a replacement job on the heater core. Also if the system has a heat control valve on the heater hose, make sure it is working! Also check the thermostat, and the temperature blend door, thats under the dash, in the heater plenum. You can flush this yourself, buy the prestone flush kit and install the tee in the heater return line. Disconnect the heater hose at the waterpump, clamp the hose in back of the tee you installed and back flush till it runs clear.20 mins or so.
As for the overheating, do the cooling fans turn on at all? Turn the A/C on, the fan should start as soon as the compressor starts, the other fan is controlled by the engine coolant temp sensor, relays, and the PCM, or computer.