After installing Calibre 4.0 I have found a weird behaviour with the viewer:
The image is like a puzzle. When I pass the cursor over the viewer window, sometimes the content change, specially when the menus are called ("puzzling" the image even more).
I am using Windows 7 64 bits, with the 32 bits program (I have tried also the 64 bit version with the same result).
This behaviour occurs with all epubs I tried, and also happen with the preview pane of the editor (although it is no so bad as with the viewer alone).