CSH,
I would first check your rad fluid anti-freeze mixture and level, if ok, next cheapest solution is to check thermostat (it s/b in a closed position when cold), if that's fine, then try flushing the system (but I agree with Artic-old rusted parts may cause more expense-there are flush kits available at CT which is basically an insert fitting for the upper rad hose allowing for the attachement of a garden hose), else, heater core could be blocked and that's expensive PITA.