Hai ragione!

Pagina principale

<script type="text/javascript" src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript" src="../scripts/menu.js"></script>
<script type="text/javascript" src="../scripts/slider.js"></script>
<script type="text/javascript" src="../scripts/jquery.nivo.slider.js"></script>

<div class="mainBox">

<div id="slider" class="nivoSlider">

[img]../images/bg_beta1.jpg[/img]
[img]../images/bg_beta2.jpg[/img]
[img]../images/bg_beta3.jpg[/img]
[img]../images/bg_beta4.jpg[/img]

</div>

</div>

Slider.js

$(window).load(function() {
$('#slider').nivoSlider({
effect:'fade',
pauseTime: 5000
});
});

Css

#slider
{
width:800px;
height:200px;
}