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userinit.exe

userinit.exe is the default executable that Winlogon runs when a user logs into Windows.

userinit.exe

runs logon scripts
reestablishes network connections and
starts the the shell (by default explorer.exe)

See also

The default value of Userinit under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon points to userinit.exe.