Notes about the NSG file extension:
Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The NSG file type is primarily associated with 'Nessie'. Nessie is an NES emulator for Windows. You can use it to play NES/Famicom games on your PC. The NSG file format changed during the Nessie development so some older NSG files may not open in newer versions of the emulator. The game files should work; just the saved state file format changed.