As mercman correctly pointed out, you can't let the A/C system sit all winter without running. If you have a vehicle where the compressor does not run when the outside temp is less than 40 or 50 degrees (I've owned two vehicles with that feature), you must manually turn on the A/C system periodically during the cold weather months or your seals can fail.
I have gotten into the habit of running my system at least twice a month during the winter for at least 5 or 10 minutes each time.
