If you get an error opening outlook (2002) stating:
"An extension failed to initialize. Can't open file: extend.dat. The file may not exist, you may not have permission to open it, or it may be open in another program. Right-click the folder that contains the file, and then click Properties to check your permissions for the folder. You don't have appropriate permissions to perform this operation." do this...
Allow Hidden Files: To view folders and files that are hidden, from my computer, click tools and then folder options. Click the view tab at the top of the window. Scroll down to the option Hidden files and folders. Click the option that says show hidden files and folders. Then click OK.
open explorer and navigate to folder:
The Fix: Make sure outlook is closed and not running. If you are viewing hidden files and folders, use windows explorer to go to documents and settings, then the folder of the user with the error. Next click Local Settings, then Application Data, then Microsoft, and finally click and go into the outlook folder. There is where you will find extend.dat. Now from here rename the file from extend.dat to extend.Old. Restart outlook and send someone an email.
PS should work in all versions of outlook. search for extend.dat