richnrockville is correct, IaskQuestions. Here's a bit more info:
Click on the "File" tab then go to "Account Settings" and select "A
In the next dialog, select the "Data Files" tab, select your active pst file (it will have a checkmark beside it), then click "Settings...":
In the next dialog, click the "Change Password..." button:
You will then be prompted to enter your old password (if you've never set one before, leave it blank) and enter your new password with a confirmation:
Next time you start Outlook, before it gets to any reading pane or calendar (anything but the password prompt) you will be asked to enter your password:
If you want to reset it so the password prompt doesn't appear, just repeat the steps above, enter the old password and leave the new password blank.