Open the computer icon. In the upper left corner click organize, then click folder and search options. Click the view tab. Change folder view to show hidden folders and search again.
In Windows 7, extened.dat file can find here : C:\Users\%username%\AppData\Local\Microsoft\Outlook\
NOTE: To easily locate the folders can also use OutlookTools (don’t worry, it’s free !)
U have to rename or delete the file and lets outlook to create new one !
Remember: This file contains information and settings for registered Exchange Client Extensions. When it is corrupted, it can prevent Outlook from loading correctly.
There is no need to make a backup of this file. Upon deletion it is automatically recreated from registry information.
Hope it may help u !