Try this
Add this to your css  display:none; so as to hide the div on the page load and then you can try your code,it works
Working DEMO
Cleaned Code
<html>
<head>
<title>Desktop</title>
<link rel="stylesheet" href="dotluv.css" />
</head>
<body>
<style>
.taskbar {
    background-image: linear-gradient(bottom, #1F5E3E 32%, #1BC27A 66%, #0CC9C9 83%);
background-image: -o-linear-gradient(bottom, #1F5E3E 32%, #1BC27A 66%, #0CC9C9 83%);
background-image: -moz-linear-gradient(bottom, #1F5E3E 32%, #1BC27A 66%, #0CC9C9 83%);
background-image: -webkit-linear-gradient(bottom, #1F5E3E 32%, #1BC27A 66%, #0CC9C9 83%);
background-image: -ms-linear-gradient(bottom, #1F5E3E 32%, #1BC27A 66%, #0CC9C9 83%);
background-image: -webkit-gradient(
    linear,
    left bottom,
    left top,
    color-stop(0.32, #1F5E3E),
    color-stop(0.66, #1BC27A),
    color-stop(0.83, #0CC9C9)
);
    width:100%;
    height:25px;
    position: fixed;  
        bottom: 0px;  
    cursor:pointer;
}
.background {
background-image: linear-gradient(bottom, #F08C11 22%, #21FCFC 61%);
background-image: -o-linear-gradient(bottom, #F08C11 22%, #21FCFC 61%);
background-image: -moz-linear-gradient(bottom, #F08C11 22%, #21FCFC 61%);
background-image: -webkit-linear-gradient(bottom, #F08C11 22%, #21FCFC 61%);
background-image: -ms-linear-gradient(bottom, #F08C11 22%, #21FCFC 61%);
background-image: -webkit-gradient(
    linear,
    left bottom,
    left top,
    color-stop(0.22, #F08C11),
    color-stop(0.61, #21FCFC)
);
    width:1350;
    height:628;
}
.window {
width:640;
height:530;
color:aqua;
-moz-box-shadow: -5px -5px 5px 5px #888;
-webkit-box-shadow: -5px -5px 5px 5px#888;
box-shadow: -5px -5px 5px 5px #888;
}
#startmenu{
    display:none;
}
.startmenu {
    background:gray;
    width:400;
    height:650;
}
</style>
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
<script>
$(function() {
    $( "#welcomedialog" ).dialog({
      height: 140,
      modal: true,
    show: {
        effect: "blind",
        duration: 1000
    },
    hide: {
        effect: "explode",
        duration: 1000
      }
    });
    $( "#internetwindow" ).dialog({
      autoOpen: false,
      show: {
        effect: "blind",
        duration: 1000
      },
      hide: {
        effect: "explode",
        duration: 1000
      }
    });
    $( "#internetopener" ).click(function() {
      $( "#internetwindow" ).dialog( "open" );
    });
$( "#startbutton" ).click(function() {
      $( "#startmenu" ).toggle( "blind" );
    });
});
</script>
<div id="background" class="background">
<div id="welcomedialog" title="Welcome">
<span style="color:#cf00ff;">H</span><span style="color:#d727c3;">e</span><span style="color:#df4e87;">l</span><span style="color:#e6744b;">l</span><span style="color:#ee9b0f;">o</span><span style="color:#f2b40b;"> </span><span style="color:#f7cd08;">a</span><span style="color:#fbe604;">n</span><span style="color:#ffff00;">d</span><span style="color:#bfff00;"> </span><span style="color:#80ff00;">w</span><span style="color:#40ff00;">e</span><span style="color:#00ff00;">l</span><span style="color:#00ff40;">c</span><span style="color:#00ff80;">o</span><span style="color:#00ffbf;">m</span><span style="color:#00ffff;">e</span><span style="color:#00bfff;"> </span><span style="color:#0080ff;">t</span><span style="color:#0040ff;">o</span><span style="color:#0000ff;"> </span><span style="color:#4000bf;">J</span><span style="color:#800080;">S</span><span style="color:#bf0040;">O</span><span style="color:#ff0000;">S</span>
</div>
<div id="startmenu" class="startmenu"></div>
</div>
<footer id="taskbar" class="taskbar"><div id="startbutton" style="width:25px; height:25px;"><img src="start.png" style="width:25px; height:25px;"></img></div></footer>
</body>
</html>