Due to decisions made by European Union, EU-versions of Office 2013 require the selection of the default file format when starting Office (usually Word or Excel) for the first time:
You can disable this dialog by creating the following registry key (can be used with group policy or logon script too):
Users starting an Office program for the first time will now not be prompted to choose the default file format. The default file format will default to Open XML (DOCX, XLSX, etc).
I've noticed that when upgrading from Office 2010, this setting does not always work properly. In this case, the user will still be prompted to select the default file format, even with the registry in place.