This is happening because when Intuit signed the older versions of the payroll reporting Excel templates they didn't use a "time-stamping" service. As a result, once the digital certificate they used expires, you will start seeing messages like the once you've reported.
To get around them, you can reduce your security settings in Excel to ignore this case, or you can update payroll to get the latest versions of the Excel templates (except that you haven't subscribed to the payroll service, so you can't.)
