Artrage typically will move the canvas when the spacebar is held down or the right mouse button is held down.
If you are seeing this happen when using the left mouse button, it is likely that something on the system is sending a spacebar or right mouse button message to Artrage. Check that the spacebar, right mouse button, tablet stylus button aren't stuck first.
Otherwise, if the user has been viewing flash content on the web ( youtube, animations ) then uses ArtRage, it's possible that the canvas will move instead of media being laid down. This is due to shockwave flash sending a spacebar message ( I'm not sure why this occurs, but we've been able to reproduce it here after watching flash content )
To correct this, try tapping the spacebar in the web browser or ArtRage, as this should reset the status of the spacebar as far as the operating system is concerned, and allow painting to occur.
I hope that helps!