When dragging a window it's easy to accidentally resize it. The problem is that Vista made the resize border larger and the title bar smaller. In XP the resize border was much thinner and the title bar wider. It gets very annoying because one needs to aim very carefully at a titlebar to avoid accidental resizing. This applies to both, Vista classic and Vista Aero.
Make the resizing area thinner like in Windows XP, and the title bar slightly wider. OR, another solution: make only the 4 corners of a window resizable, instead of edges.