IE6 and 7 combined are less then 1% of all users these days. Are you sure it is worth your time?
But anyway, I would do something like that:
<!--[if lte IE 7]> <html class="oldIE"> <![endif]-->
<!--[if gt IE 7]><!--> <html> <!--<![endif]-->
.oldIE li:hover ul {
display: none;
}