Well, I have not seen that error before, but here are some things to check on. One make sure you uninstalled the older version of NI-DAQ before you installed NI-DAQ 6.9.1. If not, go to Add/Remove Programs and remove both version of NI-DAQ and reinstall NI-DAQ 6.9.1.
Other than that I am not sure what you are doing or what application you are running or launching when you get this error. Also, NI-DAQ 6.5.1 is not supported on Win 2000.