padde's solution is right, but risen up another problem i.e.
As said by Adam (the questioner) that it is showing empty popup. Here is the complete and working solution
<html>
<head>
<script type="text/javascript" charset="utf-8" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script type="text/javascript" src="http://fancyapps.com/fancybox/source/jquery.fancybox.pack.js?v=2.0.5"></script>
<link rel="stylesheet" type="text/css" href="http://fancyapps.com/fancybox/source/jquery.fancybox.css?v=2.0.5" media="screen" />
</head>
<body>
<a href="#divForm" id="btnForm">Load Form</a>
<div id="divForm" style="display:none">
<form action="tbd">
File: <input type="file" /><br /><br />
<input type="submit" />
</form>
</div>
<script type="text/javascript">
$(function() {
$("#btnForm").fancybox({
'onStart': function() { $("#divForm").css("display","block"); },
'onClosed': function() { $("#divForm").css("display","none"); }
});
});
</script>
</body>
</html>