Drop Down Menu not transparent!!

[waldfee]

I am using a MacBook Pro 2GHz Core Duo with 2GB DDR SDRAM and OSX 10.5.3. I am confused because the menu bar at the top of the desktop is transparent, but not the drop down menu when I click on an item. This is both transparent at the Macbook Pro of a friend of mine. He has also OSX 10.5.3 installed. There is no way in system preferences to set up the transparency separately. Any ideas? (By the way: it has nothing to do with my wallpaper, elements of the picture should be visible with transparency).