I altered version 6.x in about 20 minutes to work with Windows Authentication.
What I did was make change the code field in the author table to hold the NT password, then in the common.asp file where the cookie containing the users code is read in changed it to read in the NT password which was then matched up with the code field in the database.
This is the line I changed:-
'Read in users ID number from the cookie
'strLoggedInUserCode = Trim(Mid(Request.Cookies("Forum")("UserID"), 1, 44))
strLoggedInUserCode = Trim(Request.ServerVariables("AUTH_USER"))