LabVIEW 5.1 shouldn't have any problem running on Windows 2000. I would recommend creating an email support request so we can discuss things further. Just go to http://www.ni.com/ask and select E-mail NI. In your email, please inlcude as many details about the problem as possible (e.g. does this error only occur for a certain VI, did this VI work properly on another computer, etc.) Thanks.
Matt Kisler
Applications Engineer
National Instruments