Strange Freeze - Dock, App Switching, and Context Menu died

I just had a strange freeze on my computer at work. For no apparent reason the dock, app/window switching (command-tab/command-~), hot corners, and all context menus simply died. Everything else continued to work just fine, but it was difficult to do much without using the mouse to hide apps in order to get to the one that I needed or to get to the window that I needed. I tried restarting Finder and it didn't help. I tried restarting but it wouldn't shutdown. I am running 10.5.7 on an alu iMac (1st gen). Any ideas?