In IE8, the Microsoft introduced a new mode called compatibility mode which would render the page like IE7.
You can see this button next to address bar in stackoverf
Below link was working fine in IE and it was showing menu bar properly http://line25.com/wp-content/uploads/2012/css-menu/demo/index.html
But same content hosted on intranet, all links were displayed one below other instead of menu bar. Compatibility view button was not shown in address bar.
After below change, the menu bar got displayed correctly and compatibility view button got displayed in address bar.
Tools->Compatibility view settings->Uncheck "Display intranet sites in compatibility view"
Thanks a lot for this valuable input