How many copies of the file QtCore4.dll are on your system and where are they located? This error is almost always caused by your environment picking up a copy of this DLL from somewhere other than %CEI_HOME%/apex30/win32/qt474/lib. If you open a 'cmd' shell and type:
ensight100 -no_prefs -vv
what output do you get (if any) before you see this error dialog?