When starting protection of a SharePoint farm with multiple Web Front-End (WFE) servers, you need to perform additional step.
When you run the ConfigureSharePoint –EnableSharePointProtection command you might receive the following error in a environment with more then one WFE server.
Cannot start service SPWriter on computer '.'.
In this case you first need to run the command:
C:\program Files\Common Files\Microsoft Shared\Web Server extensions\12\bin\Stsadm.exe -o registerwsswriter
Note: The administrator credentials you provide for the Windows SharePoint Services farm must be a local administrator on the WFE server
More info: TechNet forum