Solution by Event Log Doctor

I had this problem on a SBS server. Follow these instructions (from Microsoft) in order to start the service:

1. Open regedit.exe
2. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\MICROSOFT SQL SERVER\SHAREPOINT\SQLSERVERAGENT
3. Delete the value "GUID" (without quotes)
4. Start the SQLAgent$SHAREPOINT service

The service should now start correctly.

Solution by Easycom

I had exactly the same problem on sbs 2003 and this solution works!