Just in case you are trying to save and keep your auto-complete history in Microsoft® Outlook® 2010 here is fantastic blog post on .N2K file, where to find them and how to copy them. Also a good point that Microsoft Outlook 2010 does not use the .N2K file so different approach apply to this new Outlook in case you want to save the auto- complete cache .
The blog is written by a long term Microsoft Outlook Most Valuable Professional (MVP) Diane Poremsky – BLOG