ISAPI Filter, developed for IIS 6 and Windows 2003, does not work on IIS 7.5 and Windows 2008

I have an ISAPI Filter, developed overwriting the "CHttpFilter" class, in particular "OnAuthentication" method, where i replace "auth-> pszUser" and "auth-> pszPassword" to impersonate the user.
This works fine in IIS6 and Windows 2003, but when I run on IIS 7.5 and Windows 2008, the data are not overwritten and the authentication fails.
What could be the problem?, or what configuration is required for this ISAPI to work on IIS 7.5 and Windows 2008 server.