How do I know which program (or process) has a file open / locked or is using a file?

Download a free program called Process Explorer here:

Run the program and press Ctrl-F (or select Find -> Find Handle from the menu), type in the name of the file (e.g. outlook.pst), then click the Search button.

Bear in mind that the process or program that had a 'handle' on the file may have released it by the time you check...