Here are some tools that will help find out what is happening.
Use it to find with certainty which program is looking for the Outlook.pst file.
Rename the file, and when the dialog appears, start Process Explorer, then drag the icon on top of the message. Process Explorer will then indicate which program owns that dialog.
Use this to find out how this program starts up with Windows. Start Autoruns, wait for the load to be finished, then click the Find button (or Ctrl-F) and search for that program. If you want to stop it from starting with the computer, uncheck its checkbox. Autoruns will still remember it and you can always undo that action by restoring the check-mark.
If you are still unsure which product is touching the Outlook.pst file, use Process Monitor to trace all activity on the file.
Start Process Monitor up and a filter dialog will appear. Uncheck all the default filters, and create a new filter as below and click Add and OK. Leave the Process Monitor running with this filter until it finds the process that touches the file.