How to get rid of "Outlook Data File"?

I've been transferring one of our office computers from XP to 7. We couldn't get the old Outlook that the office personnel got used to, so we had to buy the latest "atrocity" MS sells. When I thought I exported and then imported the pst file I got this "Outlook data file" besides the email folder. Take a look:

Worst still, when an email is sent from an outside accounting program it is placed into that f'ing "Outlook data file" instead of our email's outbox. As a result the outbound emails sit in that data file and are never sent out.

Then if I go into account settings and try to delete that "Outlook data file" it gives me this BS:

but then when I try to set our email as default, it is grayed out:

So my question is how to get rid of this "Outlook data file" and make my emails from an outside accounting program to be actually placed into the outbox for the email account and thus be sent out?

PS. Aghhh! Why, Microsoft? Why!!!!!!!

First of all, close Microsoft Outlook, then go through Control Panel, and choose Mail and then choose Data Files, then change new default mail storage to your preference and then open and close Outlook. Come back and now you can delete old .pst file from there.

Hope that I helped.