Over time, continuous use of Microsoft Outlook can cause the program and the user folders to become congested. The performance of the program will slow down. You will find the search feature almost unusable and simple processes such as sending and receiving e-mails will hang and freeze the Outlook window. A way to increase the speed and improve the performance of Outlook is to clean out the cache, or the history, of the user folders and files.
Close Outlook if you currently have it open.
Right-click on the "Start" button and click on "Open Windows Explorer" or "Explore," depending on the version of Windows you are currently using.
Double-click on the "C:" drive in the left-hand pane to open it in the Windows Explorer window on the right side. Navigate to the "<user profile>\Local Settings\App Data\ Microsoft\Outlook" or the "<user profile>\Local Settings\Application Data\ Microsoft\Outlook" folder.
Delete the ".dat" file to clear the "Microsoft Outlook Data" file. Delete the ".fav" file to clear the favourites. Delete the ".nk2" file to clear the "Nickname Cache" file. Delete the ".oab" file to clear the offline address book. Delete the ".ost" file to clear the offline file. Delete the ".pab" file to clear the address book. Delete all or some of these files to clear the cache in Outlook. Do not delete the ".pst" file; this is your Personal Folder File that Outlook uses to open your inbox and all of your personal folders, such as your calendar items and signature settings.
Close Windows Explorer when done and restart the computer for the changes to take effect. Click on "Start" again, then "Run." If using Windows Vista/7, you don't have to click on "Run," just start typing in the "Search" box. Type "outlook/cleanprofile." This will finish cleaning the cache and open Outlook.