function MsiEnumProducts(iComponentIndex: Integer; lpComponentBuf: String): Integer; external 'MsiEnumProductsA@MSI.DLL stdcall';
 
function IsProductInstalled(szCheckedProduct: String): Boolean;
var
  index: integer;
  szProduct: String;
begin
  Result := False;
  index := 0;
  szProduct := '{00000000-0000-0000-0000-000000000000}';
 
  while True do
  begin
    if 0 <> MsiEnumProducts(index,szProduct) then
    break;
 
    if szProduct = szCheckedProduct then
      Result := True;
      index := index + 1;
    end;
  end;
 
 
function IsOfficeInstalled(): Boolean;
var
  Office,Excel: string;
  Office2k,OfficeXP,Office2k3: Boolean;
begin 
  //Office 2k
  Office := '{00000409-78E1-11D2-B60F-006097C998E7}';
  Office2k := IsProductInstalled(Office);
 
  //Office XP
  Office := '{20280409-6000-11D3-8CFE-0050048383C9}';
  OfficeXP := IsProductInstalled(Office);
 
  //Office XP + FrontPage
  Office := '{90280409-6000-11D3-8CFE-0050048383C9}';
  OfficeXP := OfficeXP OR IsProductInstalled(Office);
 
  //Office 2k3 Prof
  Office := '{90110409-6000-11D3-8CFE-0150048383C9}';
  Office2k3 := IsProductInstalled(Office);
 
  //Office 2k3 Std
  Office := '{91120409-6000-11D3-8CFE-0150048383C9}';
  Office2k3 := Office2k3 OR IsProductInstalled(Office);
 
  Result := Office2k OR OfficeXP OR Office2k3;
end;