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My Computer>Properties>Advanced>User Profiles
Click on Settings
Under "Profiles stored on this computer" will be entries for every account that has logged on to the machine (representing the saved profile associated with the local or domain account).
Periodically a profile with a name of "unknown" will appear in the list (it should not).
If you select the "unknown profile" and click on delete, the currently logged in profile will be deleted with no warning to save the contents of the "My Documents" folder.