Hello Mark,
Just wanted to make sure that the version for NI-Serial driver that you have installed is the correct one. Latest NI-Serial driver version for Win-NT is v 1.5 (http://digital.ni.com/softlib.nsf/954feaeea92d90918625674b00658b83/a87ba2c92a564e0b86256d2c007d4a51?OpenDocument)
Also, have you already gone through "Serial Hardware and Software for Windows User Manual" (http://www.ni.com/pdf/manuals/370447b.pdf)
Thank you for choosing National Instruments!
Swapnil P.
National Instruments Engineer