Ming;
By the description of the problem I can narrow down the problem to a possible cause.
My suggestion for you is to uninstall completely any version of NI-DAQ you might have there, also uninstall Labview (if you have it installed), and install the latest version of the NI-DAQ driver (6.9.2) that NI has available at its web site.
It would be good also, if you could clean up the registry windows before you upgrade the NI-DAQ version.
Hope this helps.
Filipe A.
Applications Engineer
National Instruments