Strange mouse behaviour

[Keith Clark]

My son must have set something incorrectly but for the life of me I don't
know what or even how to describe what is happening clearly.

Within a window, say Firefox, if the left mouse button is depressed, the
cursor turns into the four arrow keys and the movement of the mouse just
moves the window.  Just like it would do it the title bar were selected and
dragged around.  It happens no matter where the cursor is within the
window.

Clicking the left mouse button on a menu, like System drop down menu still
works as normal.

I hope I've described the problem clearly enough for someone to offer a
fix.

Thanks,

Keith