The following function tests to see if the WIndows Screen Saver is available, and if so, turns it on:

function TurnScreenSaverOn : bool;
var
  b : bool;
begin
  result := false;
  if SystemParametersInfo(SPI_GETSCREENSAVEACTIVE,
                          0,
                          @b,
                          0) <> true then exit;
  if not b then exit;
  PostMessage(GetDesktopWindow, WM_SYSCOMMAND, SC_SCREENSAVE, 0);
  result := true;
end;