Recently on my computer (Windows 7), some applications started freezing when I open a file dialog. (e.g., click on Save button on MS Office, click Upload button on Facebook, …)
With some googling, I solved the problem by disabling shell extensions related with Apple iCloud. I downloaded ShellExView (http://www.nirsoft.net/utils/shexview.html) and disabled ContextMenuHandler Class and Photo Stream extensions. Now it seems everything works fine.
Update 1. 2013.02.13
The above method is no longer working. I tried disabling other extensions, but the effects seem arbitrary.