If you're having trouble with your PC (read: blue screen of death), safe mode is a troubleshooting mode that can help you isolate problems. Safe mode disables unnecessary programs and drivers at startup, so you can determine whether it's a basic setting or driver that's causing your issues.
In the Windows 10 Pro Insider Preview Build 10166, booting into safe mode is extremely easy -- and you don't even have to open the command prompt to do it. Here's how:
1. Open the Settings menu and go to Update & security > Recovery > Advanced startup. Clicking Advanced startup will restart your PC in recovery mode.
2. You'll see a light blue screen with three options: Continue, Troubleshoot, and Turn off your PC. You are now in recovery mode. Click Troubleshoot to see your advanced options.
3. In the Troubleshoot menu, click Advanced options. The Advanced options menu will give you several options: System Restore, System Image Recovery, Startup Repair, Command Prompt, Startup Settings, and Go back to the previous build. Click Startup Settings to change Windows' startup behavior.
4. In the Startup Settings menu, click the Restart button to restart your PC. Your PC will now restart.
5. When your PC starts up, you'll see a light blue screen with the Startup Settings menu. You can now choose an option using your keyboard (number keys or function keys). To start up in Safe Mode, press 4 or F4. To start up in Safe Mode with networking, press 5 or F5. To start up in Safe Mode with the Command Prompt, press 6 or F6. If you change your mind and decide you don't want to boot into Safe Mode after all, hit Enter to boot normally.