procedure CurStepChanged(CurStep: TSetupStep);
begin
   if CurStep = ssPostInstall then
      if IsWindows8OrLater() then
         MsgBox('Running on Windows 8 Or Later', mbInformation, MB_OK)
         // 15 comands or call of W8-specific procedure goes here
      else begin
         MsgBox('Running on Windows 7', mbInformation, MB_OK);
         // 15 comands or call of W7-specific procedure goes here
      end;
end;

#define Windows8AndNewer "MinVersion: 6.2"
#define Windows7AndOlder "OnlyBelowVersion: 6.2"

[Run]
Filename: "Windows8-Command1.exe"; {#Windows8AndNewer}
Filename: "Windows8-Command2.exe"; {#Windows8AndNewer}

Filename: "Windows7-Command1.exe"; {#Windows7AndOlder}
Filename: "Windows7-Command2.exe"; {#Windows7AndOlder}