From what it seems, your Digital Signature software is not compatible with your system or it doesn't support XML files. In order to fix the issue, right-click the application's icon, select Properties and navigate to the Compatibility tab. Check Run this program in compatibility mode for box, select a compatible platform from the drop-down list, click Apply and then OK. If it still doesn't work, I recommend sign your documents using CryptBot web-Sign, proXSign or XSign. All three of them are fully compatible with Windows 7 platforms and can work with XML documents.