I followed the suggestion provided by Eric and used the JavaScript provided at the URL
http://www.dynamicdrive.com/dynamicindex1/hvmenu/index.htm to generate menu and its sub-menu. It is a cool web site. I am able to display my menu at different levels in the browser.
However, I stumbled on something:
I use this JavaScript in the left frame (on web page developer's left hand side and web page developer is facing the screen) to produce vertical menu in my web page.The left frame occupies about 1/6 of the width of my desktop. The other 5/6 of the desktop displays the main content of the web page. That is to say, I have frames in columns.
When I roll the mouse over the menu in the left frame, the majority view of the menu at the sub-level is blocked by the main content frame of the web page. I only see a small portion of the sub-menu. I think it is because I did not set the parameters quite right. What is the trick to have the entire view of the sub-level menu in the web page?