The Outlook adress-cache, as some might know, helps the user to find email adresses, to which he has sent an email before, pretty fast. After typing the first letter(s) of the email one’s searching for Outlook gives you several suggestions out of which you can chose the right one.
The problem is, that this cache is lost as soon as you get a new computer/Outlook. Furthermore, as this is a Microsoft program, it is sometimes removed by magical circumstances and no one knows exactly why.
The solution:
Search your PC for all nk2-files (*.nk2). You should find them somewhere in the folder AppData. The file which contains the cache is mostly called %profilename%.nk2 or Outlook.nk2 . If you want to restore the cache to your new computer/Outlook you just have to copy the right .nk2 file (if you don’t know which one is the right one take them all) into the exact same folder on your new system. To be safe for the future, copy this file on any server directory or other remote destination to have a backup in case of mysterious disappearing 
so far, greetz michael
*UPDATE*
If you have problems finding the AppData folder, your Windows probably doesnt show hidden files. You can deactivate this in the folder options.
It seems that Microsoft changed its way to save the email suggestions in Microsoft Outlook 2010. That means the restore task is extended by one more step. After searching and copying the file into the right folder ( C:\users\%username%\AppData\Roaming\Microsoft\Outlook\ ) you have to click on Start -> Run and type “outlook.exe /importnk2″. Outlook should open and the file should be included, the adress cache should be restored.
