[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program
InfoBeforeFile=d:\INNOProposal.txt
OutputDir=userdocs:Inno Setup Examples Output

[Code]
#IFDEF UNICODE
  #DEFINE AW "W"
#ELSE
  #DEFINE AW "A"
#ENDIF
type
  INSTALLSTATE = Longint;
const
  INSTALLSTATE_INVALIDARG = -2;  // An invalid parameter was passed to the function.
  INSTALLSTATE_UNKNOWN = -1;     // The product is neither advertised or installed.
  INSTALLSTATE_ADVERTISED = 1;   // The product is advertised but not installed.
  INSTALLSTATE_ABSENT = 2;       // The product is installed for a different user.
  INSTALLSTATE_DEFAULT = 5;      // The product is installed for the current user.

  VC_2008_SP1_REDIST_X86 = '{9A25302D-30C0-39D9-BD6F-21E6EC160475}';

function MsiQueryProductState(szProduct: string): INSTALLSTATE;
  external 'MsiQueryProductState{#AW}@msi.dll stdcall';

function InitializeSetup: Boolean;
var
  S: string;
  State: INSTALLSTATE;
begin
  Result := False;
  State := MsiQueryProductState(VC_2008_SP1_REDIST_X86);
  case State of
    INSTALLSTATE_INVALIDARG: S := 'INSTALLSTATE_INVALIDARG: An invalid parameter was passed to the function.';
    INSTALLSTATE_UNKNOWN: S := 'INSTALLSTATE_UNKNOWN: The product is neither advertised or installed.';
    INSTALLSTATE_ADVERTISED: S := 'INSTALLSTATE_ADVERTISED: The product is advertised but not installed.';
    INSTALLSTATE_ABSENT: S := 'INSTALLSTATE_ABSENT: The product is installed for a different user.';
    INSTALLSTATE_DEFAULT: S := 'INSTALLSTATE_DEFAULT: The product is installed for the current user.';
  else
    S := IntToStr(State) + 'Unexpected result';
  end;
  MsgBox(S, mbInformation, MB_OK);
end;