Nk2 file is a cache file which allows auto complete mail address whenever you start new email.
So basically whenever you install a new operating system or move to another computer, you should take this file with you if you want that your former auto complete recipients to work as usual.
Usually the file name will be outlook.nk2
In order to move this file from windows xo to windows 7 you simply do the follow:

1.Copy the file from windows XP under <  C:\Documents and Settings\YourUserName\Application data\Microsoft\Outlook\outlook.nk2

2.Paste the file to windows 7 under < C:\Users\YourUserName\AppData\Roaming\Microsoft\Outlook

Now you should be getting the auto complete email addresses as you where in windows XP