It happens because of z-index. I can see you have not set that, but if you use chrome you can inspect that element you refer to, lets say you right click -> inspect element on the home link and you see, the body_top will be selected in code editor.
You can prove that with setting z-index: 1 to div.menu and you will see that the links will now work, but you have to work on the design for getting this to work in the way you like, by adjusting heights and so on.
Hope it could help you.
