Well the A/C system should be looked at by a mechanic. But if your car is old and you say it takes "A lot" of power away then it could be the A/C Compressor locking up and yes this would be an A/C compressor problem. But then again either way a mechanic should look at it. I hate A/C systems I leave that to the mechanic...if its an engine problem, a leak, a belt, a alt., a radiator, a bat., a anything i fix it but if its the A/C I don't care to learn.