I have zero compression in all 4 cylinders

Actually I was coming home, parked in the driveway and when I went to start it the very next time, the sound off the motor was definately different then when it was able to start previously. The engine was about 2 1/2 quarts low on oil and the oil that was in there smelled really foul, old and burnt. There was a time or two within the past year that the cars temp gauge redlined because the cooling fan quit working. The motor was shut off imediately at those times and after replacing the fan the motor seemed to work fine up until now. I can see the pistons move up and down thru the spark plug holes when the motor is cranked, the timing belt looks fine and the timing marks are real close. I just dont have any compression in any cylinder. Where do I start?

I guess what I meant to say is the car was still in time while checking with a timing light, the belt looked good and appears to be doing what its suppose to do. What other component failures would lead to zero compression but the engine still cranks and the pistons are moving up and down while cranking it. Do I just start taking apart the cylinder head. Im hoping there are some other types of testing to determine if its valves, hydraulic lifters, piston rings, ect before I start taking stuff apart thats not necessary. I really do appreciate your time and thank you for your help.