Try this: open Computer Management on a Windows 7 computer and create a new user named CON. Then log off your current session and try logging on as CON. When the desktop appears a balloon notification will say that "You have been logged on with a temporary profile." Now log off and then log on using your regular user account and open Windows Explorer and look under C:\Users and you'll see that no user profile folder for user CON was created. Why?
Because CON is a reserved name in Windows, and if you create a user that uses a reserved name, Windows tries to create a folder with that same but is unsuccessful in doing so. Here is a full list of reserved names in Windows:
CON, PRN, AUX, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT
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Mitch Tulloch is a seven-time recipient of the Microsoft Most Valuable Professional (MVP) award and widely recognized expert on Windows administration, deployment and virtualization. For more tips by Mitch you can follow him on Twitter or friend him on Facebook.