If I may add another point. If the power supply is not regulated, the load volts will be lower than the unloaded volts. For instance, you measured 18 volts from what was supposed to be a 12 volt output, correct? Most transformers will ouput a higher voltage when they don't have a load on them. Put a load on the transformer, and the voltage should drop to a reasonable level.
NOW, place a load on the output of your power supply (A computer fan, or high wattage resistor will do) then measure again. If the voltage drops, then your power supply is not regulated and you're experiencing the poor regulation of the transformer. This really doesn't necessarily mean that you have a bad power supply. It just means that the supply isn't regulated very well.