So, is the computer booting into Windows from shut down, or not?
Using the power button to shut down the computer is generically termed a "hard shutdown" (there are other designations also) and should only be used in an emergency, such as when the system freezes and will not respond to anything, or similar events. Why? Because Windows, especially vs 10, runs system files constantly (behind the scenes) that monitor the system and provide services necessary for Windows to function. You can view some of these services that are running by entering Task Manager and clicking the Processes tab, and the Services tab. When these are suddenly forced to terminate, they may be in the middle of a calculation or process and they can corrupt and cause various problems with Windows, from not booting to some programs not working. Using the Start / Power Down command enables Windows to go through a proper close down procedure and shut down processes before automatically killing the power to the mainboard.
Your problem may or may not be caused by improper shutdown procedures, or it may be something else. To start, disable any and all Sleep or Hibernate modes as well as any screen saver modes for the monitor, just to check. If your computer still gives you problems then at least we have eliminated that as a possibility.
I have installed an external video card
Do you mean that you installed a video card inside the box using one of the PCIe slots on the motherboard, or is the card in some kind of adapter literally outside the box with a USB connection or some such thing?
Try a system reset to get everything back to square one;
> Power down (use the Start icon/Power Down command)
> Unplug the power cord from the power supply on the rear of the computer
> Press the power switch for 5 seconds (to empty the capacitors on the motherboard)
> Open the side of the case, and remove the silver CMOS battery on the mainboard. Wait 5 minutes, then snap it back in again; note the polarity and do not reinstall it backwards or the computer will do some really weird things on startup.
> Plug in the power cord to the rear of the computer, and power up.
See if that helps.