Get a cheap set of mechanic's ears and put it against the compressor body. If the screaming is coming from there, the compressor is bad (or has no oil in it). You can try an oil shot to see if that quiets it up, but you'll need to make sure and get a PAG oil if you're using 134-a. Chances are, though, even with new oil the compressor has already been damaged.

If it still screams, and you've charged it, you've probably just spread little bits of metal all through the A/C system. You now not only have to replace the compressor, but remove all parts of the entire system (evaporator, condensor, hoses, etc...) and clean them out to insure that those little shavings don't re-introduce themselves into any new compressor you install.

To test if it's the belt, just spray some belt dressing on it - if that gets rid of it, even temporarily, it's belt related.

Good luck!