The graphical Task Scheduler has its own event log, which can be viewed like this:

1. Open Control Panel
2. Double-click the Scheduled Tasks folder
3. Select the Advanced-item in the menu, and pick View Log

This log can be useful to monitor the activity of the Task Scheduler, and can help in diagnosing problems with a scheduled task.

It is possible to configure the max-size and the location of the log-file:

[HKEY_LOCAL_MACHINE \SOFTWARE \Microsoft \SchedulingAgent]
MaxLogSizeKB = 32 (Default = 32 KBytes)
LogPath = "%windir%\SchedLgU.Txt"

More Info MS KB169443

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