Outlook 2007 opens email attachments slowly…

I have a user (CEO) that has Office 2007. When he opens Excel, Word and PDF attachments, they open very slowly. I found where turning off DDE in Folder Options > File Types > “Extension” > Edit > uncheck DDE and add “%1” to “Application used to perform action:” field would fix this – AND IT DOES! Just one problem – within a week, days, sometimes hours, he is fussing again and when I look at the settings, they are back to using DDE.

Anyone know how to make these changes stick?