For Outlook 2003/2007 with Windows Vista, Windows 7, or Windows server 2008:
The location of the nk2 file is C:\Users\[User Profile]\AppData\Roaming\Microsoft\Outlook
The name of the NK2 file is identical to the Outlook profile name, with .nk2 extension.
For Outlook 2010/2013 with Windows Vista, Windows 7, or Windows server 2008:
The nk2 file is located in C:\Users\[User Profile]\AppData\Local\Microsoft\Outlook\RoamCache
The name of the NK2 file is in the following format: Stream_Autocomplete_X_AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.dat The X is the file index (usually 0) and AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA is a random 16-byte key in hexadecimal format.