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Registry: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Applets\Regedit: LastKey

The LastKey value under the HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Applets\Regedit key stores the last opened key with regedit.exe. When regedit is started, it shows the key that this value points to.
Thus, by changing the value in LastKey prior to invoking regedit, it's possible to specify which key regedit is going to show.

See also

regat.bat: A cmd.exe batch file that changes the value and then runs regedit.exe.
regat.ps1 is the same idea but for PowerShell.
op-reg-at.pl, a Perl script that changes LastKey.

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