Don't bother using it; if you really need to compress files you'll get better results from 7zip/winrar, and most files you use it on won't return you much free space since they're already well-compressed (PNGs, JPGs, video files, etc). It's extra CPU load from which you gain next to no benefit.
I LOLed so hard when I saw my brother had set his personal folder to compressed on the old XP installation; he gained maybe 1MB out of a few GBs of video files and other stuff he had.