The Windows 3.1 GPIB drivers that you are looking for are on the NI ftp site. The link below takes you to a driver download page on ni.com which will avoid having to naviagate through the ftp site - it also has documentation links on there as well.
http://digital.ni.com/softlib.nsf/websearch/73CBCD0AAEB52914862566E100749EE5?opendocument&node=132070_US
