Win2k Pro SP4 Task Scheduler service han

Since I updated Win2k Pro to SP4 last week the Task Scheduler service won’t run. It hangs on startup writing these two events to the System event log:

Event ID: 7022
Source: Service Control Manager
Description: The Task Scheduler service hung on starting.

Event ID: 7023
Source: Service Control Manager
Description: The Task Scheduler service terminated with the following error: Keyset does not exist

When I try to view a task’s properties I get this dialog:

General page initialisation failed. The specific error is: 0x80041315: The Task Scheduler Service is not running.

I’ve tried searching the kb and Technet but can’t find anything useful. Can anyone help??