N attributes mean the same thing (for some obscure reason, Microsoft prefers to display the same attribute as "N" in Windows Explorer and as "I" in the command line, at least in Windows Vista). This attribute applies only to folders and it means "do not index".
Let's check the propierties of a regular folder on Windows Explorer.
Notice there's a setting called "Index this folder for faster searching":
On folders containing the "N" attribute, this checkmark is not checked.
Let's change the folder's attributes adding
I to it .
The Windows Explorer view reflects the new attribute: