If you re-installed or did a clean instal of your operating system, there is a good bet your old data could be lost.

Normally, your Outlook data files will be in the following places:

Local Disk\users\(User name)\My Documents\Outlook Files
Local Disk\USER\AppData\Local\Microsoft\Outlook

If the .PST you have now does not contain any data, try just simply opening it with your Outlook program.

With Outlook 2010 you an hit File > open Outlook data file and navigate to the location of the file.

Hope this helps.