I clicked on administrator on the desktop and went into properties on the
Startmenu shortcut. I clicked on the Security Tab and noted that the "Group
or user names" list contains an "Account Unknown(S-1-5-21-lots of digits)"
and the litlle 'head' icon has a red-circled 'x' attached to it. What is
this? I assume it's some kind of problem. Anybody know?
The S-1-5-21-... is known as a "security identifier" or SID. Internally all
accounts are represented by a unique identifier. The names you put on the
account is just a mapping ot one of those identifiers. You can change the
name but the identifier stays the same.
Most likely what you are seeing is a remnant from an account you have
deleted that was granted access to the object you are looking at. When the
account is deleted the system does not go through and remove all access
control list entry the account was granted. When you go look at the access
control list you will see the old entry, but the system can no longer resolve
the SID to an account name, because the account no longer exists. Therefore,
it shows you the SID instead.
It is also possible that you run into this on a domain-joined computer when
it is away from the domain. A domain account has been granted access to the
object, but if the system is unable to resolve the SID listed in the access
control list to the account name when you are looking at it it shows the SID.
This could happen because the computer is unable to contact a domain
controller.
