A/C Question....

Just the other day, my air conditioning compressor started making a horrible
noise from under the hood.  It's hard to explain, but it's really loud, and
kind of sounds like an  obstructed spinning sound.  Then, if i can stand the
noise, when i turn the A/C off, theres a whining noise that gets higher and
higher pitched.  I've heard from many that it sounds like the clutch on the
compressor is seized up, or corroded....and that makes sense since it's from
Eastern Ontario, where it's known that they use a lot of salt on the roads.
I'm just wondering if anyone's ever had the experience of replacing the
clutch on the front of the A/C compressor?  I have a lot of experience with
cars, as i've never taken my car in once to have fixed, but i have no
experience with A/C compressors.  Do I have to de-pressurize the system
somehow???

Any help would be greatly appreciated !
Jeff