Navigate to: HKEY_CLASSES_ROOT
Scroll down and find the .xlsx folder. You should see three string values:
Make certain that each String’s Value is set as follows:
There are several other entries that you can edit for Excel, but if you’re only concerned with .XLSX files, this should do it for you. There are some other entries down through that folder, but you shouldn’t have to change those. If you need to, I’ve included a registry entry that you can import into yours and replace your .xlsx information.