Outlook PST File / Error - Outlook File Was Not Closed Properly The Last Time

I recently have begun having problems with my Outlook PST file. This has never occurred before over the many years I have been using the same Outlook file and I believe the problems started happening right around the same time that I put in iTunes onto my PC and started to allow iTunes to sync my Outlook contacts with my iPhone.

My Outlook file still more or less functions normally, but each time I open Outlook I get an error message telling me that the Outlook file was not closed properly the last time it was used and it much be checked for errors. It then does the check, which often it takes 20 minutes while running the check in the background, and each time it finds no errors.