Sometimes when you use the fantastic SyncToy powertoy from Microsoft, you get a dialog box with this error message:
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SyncToy
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Exception during run: The event log file is full
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OK
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The fix is easy: open the Start menu, click on "Run...", and type "eventvwr". This will open the Event Viewer and there you can right click on any of the available logs to clear all events. Before you do that however, I suggest browsing these logs to see if any real problems are listed there... actually, you should do this regularly anyway!