Back in Outlook 2003-2007, Microsoft used to save your AutoComplete Contacts in a cache file with the extension NK2. When moving from one PC to another, we used to just copy the file to the new machine and Outlook automatically loaded the information.
In Outlook 2010 things changed, and now the data is saved as "Suggested Contacts" inside the Personal Folder data file.
To import .nk2 files into Outlook 2010, follow these steps:
The .nk2 file is renamed with a .old file name extension on the first start of Outlook 2010. Therefore, if you try to re-import the .nk2 file, remove the .old file name extension.