By default QGIS uses the Qt QPlastiqueStyle for all its windows which makes QGIS render all controls like this:
However it can be changed in order to make it fit in with system style, as in use Windows style. Running the following commands in the python console:
from PyQt4.QtGui import QApplication QApplication.setStyle("windowsvista")
will change QGIS to render controls in windows style:
It’s quite hard to notice many of the differences but just try it you will see them
However there is one limitation. QGIS will not remember that setting as the compiled binary forces QGIS running on Windows into QPlastiqueStyle.
I have started working on a patch to give the user a choice in the options which control rendering style they would like to use. I think choice in these situations is good because I have grown to like the QPlastiqueStyle, mainly because I have used it so much , but other people I know prefer the Windows Vista style controls.