Hello,
There is a good page on our web site that has troubleshooting pointers for troublesome installations of our plug and play serial boards. The link for this page is here:
http://www.ni.com/support/serial/trbpcint.htm
I would also suggest doing a completely clean installation of the driver and the serial card. By this, I mean uninstalling everything (driver and card) and ensuring that you install the driver and then the serial hardware.
I hope this helps! If you have further issues, please let us know.
Anthony Bacak
National Instruments
Applications Engineering