Copy the .nk2 file from your old installation of Office. Under Windows XP, it is located at "DriveLetter:\Documents and Settings\Username\Application Data\Microsoft\Outlook," where "DriveLetter" is probably your C drive, depending on your computer's configuration. You will need to enable viewing of hidden folders through "Folder Options" to find this location. The file will have the same name as your email profile, which may simply be called "Outlook.nk2."
Paste the old .nk2 file to the new location used by Office 2010. If you are transferring to a different computer, a USB drive is a fast way to transfer the file. Office 2010 stores its .nk2 file in the folder "%appdata%\Microsoft\Outlook."
Open the Control Panel. Double-click the mail icon. Click the "Show Profiles" button. The profile name for the .nk2 needs to match the name of your Outlook profile. By default, the profile is named "Outlook." If you are using a different profile name, you need to rename your .nk2 appropriately.
Click the Start menu and select "Run" if you are using Windows XP. On Windows Vista and 7 you can use the box that says "Search programs and files."
Enter "outlook.exe /importnk2" in the box. Outlook will launch and the autocomplete list in Outlook 2010 should be populated. Test it by creating a new email and entering an email you know. Outlook 2010 stores the autocomplete list differently from previous versions and your imported .nk2 file will be renamed to "yourprofile.nk2.old."