Lately I've been having a lot of trouble shutting down and resetting my iMac. I'm running 10.4.8, and what happens is when I try to reset or shut down, it's like everything closes, but my desktop background is frozen on the screen and all I have left is the mouse (which can move freely). It'll sit there for as long as I have patience to wait (I've even stepped away for quite a few minutes and it was still there when I got back). I'm left only to hard shut down by holding down the power button. I've had to do this several times. Why is this happening and what can I do to fix it? It's incredibly frustrating.