John,
Always helpful to have a little more information such as OS, client?, Server, if so, what kind. Domain vs P2P, etc.

OK, when you open your Task Manager, do you see anything like "Stop if computer ceases to be idle" option. If so, see below.
Is the Task Engine as a process working to allow Task Scheduler to run? Task Engine = taskeng.exe. Are there any scheduled events that are not running or can you correlate the time of the error message with an event? So, is taskeng.exe under image name in Task Manager?
What errors are you seeing in Event Viewer under Microsoft System? If anything to do with task scheduler or taskeng.exe, further research it.
It is located in System32, so you could just kill it, but I wouldn't recommend it as your task scheduler may not working. Of course, if you just renamed it, you could always put it back. But, that may be a lot of events.
http://support.microsoft.com/default.aspx?scid=kb;en-us;950363
If you do have the "Stop if computer ceases....option," run the above.