Atlast My Self found the solution. The solution is jquery Slide will work only after the sliding element completely loads. Hence the slide function must be given in the callback method of the load function.
$(window).load(function(){
$('#divhome').load('home.html', function() {
$('#divhome').slideDown(2000);}
);
$('#divgallery').load('gallery.html');
$('#divabout').load('about.html');
$('#divcontact').load('contact.html');
});