Microsoft itself does not support IPv6 for Windows 95/98 or NT4. There is however an IPv6 implementation from Trumpet by the name of Winsock which can be installed on these Windows versions (and even on Windows 3.1). The software is not free. There is also a try-out version availabe for a 30 day trial period, downloadable on their website.
Unfortunately the authors of this Howto have no experience with using this software and would like to refer the interested reader to the documentation that comes with Trumpet's Winsock implemenation itself.