Longhorn is to do something that *nix systems have done since the beginning. User based pemissions. A system like this can increase security as it allow/prohibits actions based on a users permissions. Though the great downfall still lies in the end user, this should help slow the spread of malware/viruses etc.
http://www.pcworld.com/resource/article/0,aid,120314,pg,1,RSS,RSS,00.asp
