When I try to open Outlook I get the following message: "Cannot open your default e-mail folders. File access is denied. You do not have the permission required to access the file C:\users\ . . . . . \Documents\Outlook Files\ . . . . .pst."

When I try to open the .pst files I get a message that they can only be opened within Outlook. So, now i'm stuck because I can't open Outlook. It started happening when I downloaded Windows 10. But now that I've reverted back to Windows 8.1, I still get the message.