Try either:
word-wrap:break-word; /*use this in the #zd-nav .zd-sub-nav class*/
or...
width:100%; /*instead of setting the width to 73px*/
you could do height:100%;
also.
It's a hasLayout issue that Microsoft invented. Found the info here: http://zoffix.com/css/ie/haslayout.shtml
Hope this helps...