jquery function does not load when using scriptmanager

Question

I currently have the following code that display a jquery growl when the page loads.

<script>
    window.onload = function () {
        var msg = "My alert message";
        jQuery.jGrowl(msg, { life: 5000, theme: 'redgrowl' });
    }
</script>

I would like to be able to call a jquery growl function from codebehind, I use a scriptmanager but have so far not been able to get it to work. I have tried this..

var sb = new StringBuilder();
sb.Append("    window.onload = function () {");
sb.Append("        var msg = \"Hello world\";");
sb.Append("        jQuery.jGrowl(msg, { life: 5000, theme: 'redgrowl' });");
sb.Append("    }");

ScriptManager.RegisterStartupScript(this, GetType(), "jgrowlwarn", sb.ToString(), true);

But that does not show anything at all. I have the current code that load on window.onload on the bottom of my page just before end of body, and the jquery.growl.js is in the head tag. What could be wrong?