I am not really worried about this, just curious. My OS is win7 and I just finished copying a few hundred music videos from some DVD's to my PC.
I decided to look at the properties of one of the videos and this is what I find under the security tab in the Group or user names box.
Account Unknown(S-1-5-80-random numbers...
I looked this up and found several bits and pieces of info but nothing to clearly describe what this is or why.
These videos were downloaded and burned to DVD's on this same PC but an older OS, Win XP and a different user name.
I am guessing this account unknown is because of the permissions granted by windows media player for the streamed info pertaining to artists and the names of their songs but I don't know for sure. Any info is welcome...
Windows accounts have names like "johnsmith" associated with them, but internally Windows tracks their security using an internal "GUID" (Globally Unique IDentifier). Think of it like the US Government - so many people have the same name that they have to assign you a Social Security number to keep everything straight.
When you install a new version of Windows and create a new account for yourself, it assigns your new account a fresh, different GUID even though you use the same name. When you then copy a file from another system that has security associated with it, the old GUID in the security descriptor doesn't exist on your new system, so you get the "unknown account" message that you saw.
It's not a real problem, and if you want to fix it then just right-click the file, go into its properties and security settings, and change the security settings for it.
If all the files are in one folder then you can easily select them all and apply the security changes en masse. Not that you have to do that - just so as you know it doesn't have to be a one-file-at-a-time kind of thing.