Search the boot drive

Use "BootDir" value in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup:

uses Registry;
...
procedure TForm1.Button1Click(Sender: TObject);
begin
  with TRegistry.Create do
  begin
    RootKey:=HKEY_LOCAL_MACHINE;
    if OpenKey(
      'Software\Microsoft\Windows\CurrentVersion\Setup', 
      False) then
    try
      Edit1.Text:=ReadString('BootDir');
    except
      MessageDlg('Can not go to this handle',mtError,[mbOk],0);
    end
    else
      MessageDlg('Error registry reading',mtError,[mbOk],0);
    CloseKey;
  end;
end;