PopMessenger was designed to have compatibility to Windows version up to Vista. This means that you'll encounter stability issues if the application is used in any version of Windows higher than Vista (Windows 7 or Windows 8). Fortunately, Windows 8 offers backward compatibility. Proceed to install the file then close it down without going further. Windows will automatically detect this and will offer a compatibility assistant wizard. Select to fix it automatically then start the application again. With a bit of luck, you'll be able to install PopMessenger in Windows 8. The problem will remain if the application uses DLL files that are not designed for Windows 8, as you will encounter additional errors which can't be fixed with compatibility settings.
