A nice summary of the history of the letter Q:
During the Old English period, we didn't use Q in English: we wrote, for example, CWICU for 'quick' and CWEN for 'queen' (Old English, like Latin, preferred C for the /k/-sound instead of K). But then the French-speaking Normans conquered England, interrupting the English literary tradition, and, when English once again began to be written after the Conquest, a number of French spelling conventions were introduced, including the business of always writing Q for the /k/-sound when the next letter was U. And we're still stuck with it.
(Via aldiboronti at Wordorigins.)