This is a, slightly hackish, pure CSS implementation of a fixed table header.
It is implemented in HTML5 and CSS3 and is tested in FX 25, Chrome 31 and IE 10.
This is a wordpress 3.0 + menu only may be converted but you need to look and add your own code for other systems
check out the complete write up here http://stackoverflow.com/questions/5839623/mouseover-remove-class-mouseout-restore-class/5844923#5844923