When you attempt to create a new scheduled task, you receive the subject error?
NOTE: Existing tasks do not start. The error code is 0x80090016-keyset does not exist.
The problem is likely the result of a corrupt %SystemRoot%\System32\Mstask.dll library.
You may have a good copy at C:\WINNT\system32\dllcache\mstask.dll.
If not, use the Recovery Console to copy the file, from the latest service pack or CD-ROM, to both folders.