[PyQt] How can i load the codec plugin dynamically?

[DDGG]

Hello friends!
I want load a codec plugin at the runtime of my application, because
it could not load automatically by PyQt itself.
I add three lines in my source file, but it failed.

plugin = QtCore.QPluginLoader("C:/Qt/4.3.1/plugins/codecs/qcncodecs4.dll")
print plugin.load()       # result is false, why?
print plugin.errorString()  # application crashed in this time!

I needs your help, thanks a lot!
(mine: PyQt: PyQt-Py2.5-gpl-4.3.1-1.exe,  Qt: qt-win-opensource-4.3.1-mingw.exe)