Hi Michael,
Sorry to hear that you are having trouble.
Heres a few steps to try and narrow down the problem.
Download and instal NI-Serial 1.71 from here - Note, log into the website to see the download link.
http://digital.ni.com/softlib.nsf/954feaeea92d90918625674b00658b83/e1e0b681afe1635786256f640054da71?OpenDocument
Instal the software and reboot.
Once powered up insert your USB-232/4.
This should be all you need. NI-Serial 1.71 does indeed support dual processors under Win 2000.
If this is still not working then run the Serial Troubleshooting Wizard, either from your install directory or better still from Measurement and Automation Explorer>Help>Troubleshooting>NI-Serial Troubleshoting wizard.
If you could post exactly where the error occurs and any additional error info that would be great. Your system specs would also be handy.
All the best,
Matt Poole | Applications Engineer | National Instruments