Even though you have applied SP4, you continue to experience the error from Microsoft Knowledge Base article 329962 - Task Scheduler may create corrupted job files.
This behavior will occur if Protected Storage becomes corrupt.
To resolve this behavior:
1. Move the contents of "C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\S-1-5-18" to another folder for safe keeping, causing the S-1-5-18 the sub-folder to be empty.
2. Shutdown and restart Windows 2000.
NOTE: The restart should cause Protected Storage to be rebuilt and your scheduled jobs should run.
NOTE: This solution will also work if you experience these same symptoms on Windows Server 2003 or Windows XP.