Account Unknown in all Public Desktop icons
 

When viewing the Group or User Names in the Security tab of Properties in any
icon stored in the Public Desktop folder, 'Account
Unknown(S-1-5-21-63554606-914774692-129410296-1000)' is displayed with a red
question mark in addition to the 'head' icon. Also, there is no sign of the
Users group, but for some reason the INTERACTIVE group is listed (which I'm
not very clear about). SYSTEM is also listed, along with the Administrator
account and group.

If I move any icons to an individual account's desktop, it lists SYSTEM, the
Administrators group and the account to which the icon has been moved, which
is what I would expect.

Why are Public Desktop icons displayed like this and how is this rectified?