javascript popup menus on block navigation module

I need to develop a web site which will need to have a right block for navigation where there will be a certain number of links.
I need to add a javascript code to each link so that onMouseOver I will get a popup menu that will give me sub-navigation options.
Is that easily achieavable under drupal? Would I have to write a block module for it and the js on a separate file? How would I integrate the two things and how can I go on about it?