This file helps OL to cache registry entries for 3rd party add-ins…. To get rid, open OL click Help menu and select detect and repair & start diags by clicking button at the bottom of the page. After completing close OL and probably the prob gone ! Else u can rename that file by searching with the same name(extend.dat) in search box, after getting this file right click to rename the file as extend.old (but don’t delete it). Start OL.
When you ask for help in solving an error, it is a good idea to post the complete error message.