The use of the spelling "magick" originated with Aleister Crowley and the Order of the Golden Dawn. They used the pseudo-archaic magick-with-a-k to mean ceremonial or high magic as opposed to practical magic. In some traditions of modern Paganism, the magick-with-a-k is still used today, however, just as many traditions choose not to use it. Some, in fact, find it a wee bit pretentious to use unless you happen to actually BE a ceremonial magician (or would that be "magickian"?).
Because I'm someone who really needs consistency in my life, when I took over this site a year and a half ago, I made the decision to use magic-with-just-a-c-on-the-end in all pieces of content that make references to magic. The only time you'll see me use magick-with-a-k is if I'm directly quoting a source which spelled it that way. It's just a matter of personal preference, and anyone who really likes to spell it with a K is more than welcome to do so.
At any rate, I hope that clears up any confusion that might be lurking out there. For more info on things magically delicious, you can read here: Glossary - Magick