Hold the Shift key while clicking Restart from the Power menu. Select Troubleshoot > Advanced options > Startup Settings > Restart , then press 5 or F5 for Safe Mode with Networking.
Here is a solid guide to removing it and reclaiming your system. How to Remove System Security Virus Infection
Safe Mode loads only the essential files and drivers, which usually prevents the rogue software from starting up. Hold the Shift key while clicking Restart from
Before doing anything else, (unplug the Ethernet cable or turn off Wi-Fi). This prevents the virus from communicating with its home server, downloading more malware, or spreading to other devices on your network. 2. Boot into Safe Mode with Networking downloading more malware