According to a manual that was able to dig up online, the latest version of FormFast Form Designer does indeed work with Windows 10. Unfortunately, as far as version 5.1 is concerned, there's no information regarding operating system compatibility, so I would recommend updating the application. In case that's not an option, you should try contacting the company via email: [email protected] as I'm sure the support staff will be able to answer any compatibility-related questions that you may have.