Beach Ball seems to stop all applications

This has happened to me twice. I go to an app like Photoshop and try to do something or save a file and I suddenly get the dreaded beach ball. And it won't go away. I go to Force Quit and see it's not responding. So I eventually have to force quit. But a couple times lately it when this happens it starts happening with many of the rest of my applications. If I try to shut down or restart nothing happens. So I press the power button, turn it off, and then start it again. And all is fine.

Now I do leave my computer on for days at a time. So what's happening and how can I fix it. I think it's something to do with the system rather than any one application. The iMac's a little over a year old.