In Windows 7, the taskbar buttons are too low (or the taskbar is too high - I don't know what is true). The taskbar with Classic design and classic taskbar settings looks like:
As you can see this looks "unnatural" or "buggy" because there are many grey pixels under the taskbar buttons.
On the other hand, if I resized the taskbar to hold two rows, everything looks fine:
It seems that the problem is related to the userprofile (which was recovered from a previous installation).
I tried resizing or changing settings back and forth: no change.
Therefore the answer to the question could be the answer to:
"Where does Windows (7) save settings of the taskbar geometry?"
In this case, I could delete this data so that default settings are restored.