If you see white boxes behind labels in the Preferences window (as in the screenshot below) then there are two possible fixes, depending on the exact cause of the problem.
First, ensure you have the latest version of Opus since version 188.8.131.52 included a fix for an incompatibility with certain unofficial Windows themes/visual styles.
If you still encounter the problem it is probably because WindowBlinds compatibility mode is turned on. Turn it off and things should go back to normal.
(Some people turned on the WindowBlinds compatibility option when running Opus 8 on Vista to correct issues with window titlebars. This is no longer necessary in Opus 9.)