Odd issue on my Windows 7 Professional (32-bit) system.
If I click on the Start 'orb' then navigate to 'All Programs', then navigate to any of the folders that appear at the bottom of the list (like Accessories) and left-click, the folder contents expand with no issue. If I right-click, the context menu appears for half-a-second or so, then the popup goes away and the entire start menu dismisses.
I'm not sure how to debug this issue - I'm considering using Autoruns to try and disable things hooked into the shell one-by-one. Is there a way to use a tool like Process Explorer to narrow down the process that's actually dismissing the menus?