FAQ - Frequently Asked Question list - Answer

Question: How to make a new PST file the default file in Outlook?

Answer: You should:

1. Open Start | Settings | Control Panel | Mail
2. Use the Delivery tab to select the corresponding PST file in the Deliver new mail to the following location: combo box and confirm moving the service folders (if necessary).

Thus, the new pst file will be the file used in Outlook by default.