It's there for a longer time, but I still get many requests about it's avaiability... So, here it is: The Windows Vista Security Guide!
http://www.microsoft.com/technet/windowsvista/security/guide.mspx
Direct document download:http://www.microsoft.com/downloads/details.aspx?FamilyId=A3D1BBED-7F35-4E72-BFB5-B84A526C1565&displaylang=en
Urs