If you are using Microsoft Outlook, all of your email is stored in a Personal Folders file (.pst), which tends to build up in size over time. If you want to save a little bit of hard drive space and maybe speed up Outlook, you can compact that file easily.
Of course this tip isn’t very useful for experienced Outlook users, but we like to cover everything helpful even if it’s fairly basic.
Compacting Outlook Data Files
From the Outlook main window, choose Data File Management from the File menu.
The Account Settings dialog will open up on the Data Files tab. Highlight Personal Folders, which should be located on the local drive, then click on the Settings button.
Now with the Personal Folders dialog box open, click on Compact Now.
A message indicating compacting is taking place will pop up then disappear when done. Now we can close out of the remaining windows.
This will work in both Outlook 2007 and 2003. If you notice Outlook becoming slow try out this quick fix.