[valdet]

#tab li 
{
display:inline;
        float:left;
	margin:0;
	padding:0;
}
#tab a 
{
	background:#000000 url("forum/bright_066w.gif") no-repeat right top;
	margin:0;
	padding:0;
	text-decoration:none;
	border:0;
	display:block;
        float:left;

Those are most likely causing you problems.

I know there is a limitation in CSS because there is no float:center property, so my bet is that, in order to have that menu centered, you need to have the whole div inside a table.