Text typing by itself, loads of X/boxes appear.

When I choose the Type Box tool, and click in the document to create a Type Box or simply place text, as soon as the box is created, it fills with X's/squares and goes off on it's own continually typing the X/square. I've tried changing font, but it always defaults back to Myriad pro as it's typing. Tried fiddling with some preferences, tried shutting the program and restarting - not sure what to do. Please help!