Moving Outlook's Autocomplete Addresses
For pre-Vista machines, the NK2 is stored in C:\Documents and Settings\{UserName}\Application Data\Microsoft\Outlook. What's interesting is that this is not the place where Outlook PSTs are stored by default. For pre-Vista machines, that location is C:\Documents and Settings\{UserName}\Local Settings\Application Data\Microsoft\Outlook, which is a slightly different path that meanders its way through the Local Settings folder. On Vista machines, as you probably know, the path for profile information is quite a bit different. Thus, the ultimate resting place for the NK2 is different as well.
For Vista, you'll want to deposit your NK2 into the C:\Users\{UserName}\AppData\Roaming\Microsoft\Outlook folder. Once there, Outlook will begin using that file again to populate the drop-down list.
While looking for this file, you may also want to look through it as well. If you've been annoyed by the number and uselessness of certain addresses in your drop-down list, you can eliminate them from this file to remove them from your drop-down list. Doing this, however, is no trivial task. You can't just open this file in Notepad. To edit your NK2, download a freeware tool called NK2.Info. This tool shows you the addresses stored in the file and allows you to add and remove addresses as necessary.