Windows Authentication and IE Passing Credentials

This isn't an issue with IE not passing credentials, this is a stranger problem.  For the majority of our intranet users, Windows Authentication works correctly and IE passes the proper credentials, domain\username.  For a handful (maybe more) IE appears to be passing the ASP.NET process account or the Administrator account.  Occasionally the accont shows up as Network Service, much more often it shows as Administrator, neither with the domain name.  Any ideas what may be going on?  Doesn't seem to matter whether it's IE 7, IE 8 or Firefox, as long as they pass credentials they occasionally pass incorrect ones.  Clearing caches and saved passwords doesn't fix this.  Currently appears to be only specific users, it either always works for the user or never does.
Incidentally, if IE doesn't pass the credentials (take the domain out of the Intranet zone) and the user logs in and saves their password, it works ...