[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
#ifdef UNICODE
  #define AW "W"
#else
  #define AW "A"
#endif

const
  MAX_PATH = 260;
  ERROR_SUCCESS = 0;
  AutoCAD2016PC = '{5783F2D7-F004-0000-5102-0060B0CE6BBA}';
  INSTALLPROPERTY_INSTALLLOCATION = 'InstallLocation';

function MsiGetProductInfo(szProduct: string; szProperty: string;
  lpValueBuf: string; var pcchValueBuf: DWORD): UINT;
  external 'MsiGetProductInfo{#AW}@msi.dll stdcall';

procedure InitializeWizard;
var
  RetVal: UINT;
  BufSize: DWORD;
  AppPath: string;
begin
  BufSize := MAX_PATH;
  SetLength(AppPath, BufSize);
  RetVal := MsiGetProductInfo(AutoCAD2016PC, INSTALLPROPERTY_INSTALLLOCATION, AppPath, BufSize);
  if RetVal = ERROR_SUCCESS then
    MsgBox(AppPath, mbInformation, MB_OK)
  else
    MsgBox('Function failed. Error code: ' + IntToStr(RetVal), mbError, MB_OK);
end;