Also, a good general troubleshooting step, if PS is the only program doing this to you, is to quit Photoshop, then move its preferences file out of where it is. Its location depends on your OS and the version of PS you're using. See
this page on Adobe's site for the location for your version/OS.
Don't trash the file just yet. When you restart PS, it will create a new, fresh prefs file, but without your custom settings. If all is well, you can now throw the old one away. If this didn't solve your problem, you can always shove it back where it belongs.
Photoshop's prefs file is famous for getting corrupted. I keep a copy of what I know is a good one, with all my settings, so when it goes south, I can simply replace it with the good one.