This can be caused by quite a few problems. First, the PC preferrably is on that has been tested and approved by the manufacturer. The most common cause however is problems with the GPIB controller configuration in the PC. If you not allready tried:
1) Uninstall the National instruments driver in add/remove programs.
2) Shut down the PC
3) Remove the GPIB interface card in the PC
4) Reboot the PC
5) replace the Install the national instruments folder from the MS Workstation CD
6) Shut down PC
7) install GPIB interface card
8) reboot PC (the "found new device should appear at some point)
9 Re-install MS Workstation