Hi,
According to your description, I understand that explorer.exe not running while system start, however manually run explorer.exe successful.
If I misunderstand your concern, please do not hesitate to let me known.
Do you modify system settings or install applications before this issue arise?
Please try clean boot, also open Regedit and switch to
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon, ensure the value of shell is "explorer.exe".
Note: please backup registry firstly before modify the value of it.
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Allen Wang
TechNet Community Support
