I have Windows XP, and I have many maintenance tasks controlled by the Task Scheduler.  Several are supposed to run each night.  This worked fine for about a year.  Now, about every other day, when I log in, I am confronted with the message: Several Scheduled Tasks did not run because the Task Scheduler is turned off".  Sure enough, it missed last night's tasks.  If I then change the schedule to run a task a minute from now, it runs.  I then change it back to the original time, and it runs then, too - for a while?  What gives?  And how could you turn off the Task Scheduler anyway?

   Task Scheduler is started at startup and must be running in order to run the scheduled events.
   It could be that for some reason it was not started at startup in those situations or that it was closed before the scheduled event occurred.

You're absolutely right.  But how could it have been closed, and how could it be that it didn't start at startup?  One possibility: I used Speed Startup to trim my list of running programs.  Any idea what the name of the executeable for the Task Scheduler is? Perhaps I removed it by mistake... but that doesn't explain why it sometimes works
and sometimes doeasn't.

   I think that it is called mstask.

I found it listed as Task Scheduler under Services in the control panel.  It is turned on, and it has been running lately.  The next time I get an error message, I'll look there again.  I'm sure this will not teach me anything.

Look at the event viewer under administrative tools in the control panel.
You can view system errors and application errors showing date and time of each event.
There should be some information there regarding this problem.