Several problems...can anyone help?

I have a 1996 automatic Pontiac Sunfire. I am mad at it! I originally brought my car to someone to fix the head gasket because my engine was getting really hot. He fixed that, but maybe over fixed it. Now when I drive when it is cold outside my engine will cool down the faster I go, then goes up to normal temp when I am on regular 40 mph roads.
When I go on the highway when it is cold outside (40 degrees and lower usually) the gauge goes down, sometimes to the bottom.
Secondly, since I got my car back it is now making a whining noise (like a morning dove but consistant) when I am going 30 mph and lower, it is at its loudest when I am stopped. I turn off my heater to see if it would stop, but it does not!
Lastly, my check engine light is coming on, steady. What is wrong with this car!?

Go to autozone or some place related and have them check your computer to see what the light means... its a free check.. the check engine light doesn't always mean a major problem, might be something like your O2 sensor is going bad... As far as the noise, have your belts checked that might be causing your problem.. You may just need to get new belts or have them tightened... And it sounds like you just got a messed up thermostat.. Unless your car dies or hesitates at acceleration I'd say its nothing major.. Just have your belts, thermostat and computer checked out, chances are its a minor problem thats easily fixed

thanks, i am getting it checked out today.