Can't delete .olk files. Why?

I'm cleaning out my old computer to give to my wife and I've cleaned out almost every file EXCEPT for the .olk files. Those are Microsoft Outlook files representing emails and contacts.

I'm able to drag them into the trash and then empty the trash. I can see the progress bar, and then the trash is empty. But when I restart the computer and go to "all my files," there they are again!

Any ideas about what I'm doing wrong?