If there is a prompt that the graphics card driver is unsigned, it means that the current driver has not passed the technical verification of Microsoft, and it is not the official version of the driver. You need to install the WHQL driver verified by Microsoft, go to official website, NVIDIA, search and download the N card driver according to the model of the graphics card and the corresponding Windows7 system, uninstall the original graphics card driver, restart the computer, and then install the newly downloaded WHQL driver with Microsoft digital signature.