Hello,
Those error codes are usually generated when a session is not valid, the instrument is not at the primary address specified, or the instrument is not on. (You can paste error codes into the utility at Help>>Explain error code in LabVIEW) Make sure you have only one driver installed. If you have LabVIEW 6.0.2, NI-488.2 ver 1.6 installs with it as part of the complete install.
If you have LabVIEW 6.1 or later, try installing the GPIB-USB-B on another machine. If it does not work on the other machine, then it may need to be repaired or replaced.
Ray K
NI Applications Engineer