Never10 won't get rid of the nag. It can prevent Windows 10 from installing, and remove any hidden Windows 10 installation files that Microsoft may have downloaded to your hard drive. But it won't block any "normal" Windows updates, and Microsoft considers the Windows 10 nagger to be a "Recommended" update.
To get rid of the nag, you need to uninstall the Windows Update that installs the nag, which is KB3035583. You uninstall it through Control Panel. Then you need to run Windows Update manually and tell it to "hide" KB3035583 (right-click and hide). If you hid KB3035583 in the past, you have to do it again because Microsoft made changes to it, which "unhid" it.
C:\WINDOWS\systerm32\config\systemfile\desktop is unavailable... I just booted up my Cyberpower with OS 8.1 and this is the message I got.
The power supply may be starting to fail -- not providing enough power to initialize the hard drive during a critical phase of booting. Sometimes letting the computer run awhile at the "unavailable" screen before rebooting will allow the power supply to charge up enough to succeed on reboot, but this doesn't fix the problem. If this problem booting happens again, and continues to happen, you may want to consider replacing the power supply with one of equal or better quality.
If you are interested in why power supplies fail, there is a discussion here
http://forum.allaboutcircuits.com/threads/what-causes-a-computer-power-supply-to-fail.13973/Reasons for failure seem to be heat from dust accumulation, fan wearing out, bad capacitors, ... If you replace the power supply, be sure to get a good one.