Hello
My answer is a bit late, but it might still be relevant:
Read these posts (http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=249624) and (http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=360023) and download the latest version of our Open Souce Asp.Net Security Analyser (it also contains the IIS 5.0 security guide with detailed instructions on how to solve this problem)
Read this post to see what I have done so far to call microsoft's attention to this problem "When will Microsoft take Asp.Net Security seriously?" (http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=370723)
Best regards
Dinis Cruz
.NET Security Consultant
DDPlus (www.ddplus.net)