Hi,
I couldn't find *ANY* references to the HP 8539A network analyzer on the web. Not even HP/Agilent had it listed as a product. Perhaps you typed in the wrong model number. Anyway, if the instrument is a GPIB capable device, you should be able to talk to it fairly simply using LabVIEW. We have thousands of Instrument Drivers listed at www.ni.com/idnet. There may be additional ones at the manufacturers' websites.
If an Instrument Driver doesn't exist, you can request one at http://zone.ni.com/idnet97.nsf/instrumentdriverrequest/ and we will develop them based on how many requests we receive for it.
In general, if you have the programming manual for your instrument, its not too hard to write your own drivers.
Hope this helps.
A.S.
Anu Saha
Academic Product Marketing Engineer
National Instruments