Ther could be 2 reasons for this error,
1. That error is a VISA error which usually means you are incorrectly addressing the VISA resource. If you open up the VISA Interactive control (NIvisaic.exe) in the VXIpnp directory of the machine that is directly communicating with your HP instrument, you should be able to see how to correctly address the instrument with VISA and test the communication a little. Make sure you are not opening multiple references to the same device by using the older VISA Open inside a loop.
2. I found knowledge base (KB) entries that talks about your question. If you go to ni.com >> Support >> search (option 3) select Knowledge base from the dropdown list >> and type the following "old visa open" in the search box, you will get them.
AE KB 21DCA12L : How Do I Use the LabVIEW 5.x "VISA Open" Function in LabVIEW 6.0?
AE KB 2CLJ1G8V : VISA Open and Close no longer necessary in LabVIEW 6i.