/S option for regedit is a silent mode.
You either create a shortcut for regedit.exe
and then modify Target line on Shortcut tab of Shortcut properties
into something like that
Code:
C:\Windows\regedit.exe /S "C:\yourfile.reg"
and place it in Startup folder in Start menu
or create a batch file with the same content and put it in that Startup folder.