Scheduled Tasks fails to start with Event ID 101

A task might fail to start.  Checking the Task's history you may find an  Error with

Log Name: Microsoft-Windows-TaskScheduler/Operational
Event ID: 101
Task Category: Task Failed to start.
Source: Task Scheduler
OpCode: Launch Failure

As cause for this is often a setting on the General tab for the task.

Even if an alternative user is selected to execute the task you need to select the  "Run whether user is logged on or not" radio button.

For more event IDs check http://technet.microsoft.com/en-us/library/dd363625(WS.10).aspx