regional settings

My computer went into a complete default. Now when I try to open my accounting program it tells me that I need to change my short date format to default.When I try to change the setting in regional settings from the drop down box when I hit apply it always tells me
" One or more of the characters I have entered in this field are invalid Try typing different characters"
How can I change the settings
Thank you.

 

This is an application issue so I'm moving this to that section.

It would really help if you specified what application you are using.