You can check your system RAM to see how much percentage you've got left (in system information). Some programs run even in the background so you have to be careful (shows up in taskbar near clock)
I've had this problem when connecting to the net so disconnect and try again. Run Scandisk (usual routine)
If all else fails - a good restart usually does it.
If not, you've just earned yourself a new doorstop.
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Greetings Mortals. My Name Is Wah, Arch Lich Wah.