1. Click on Start > All Programs > Accessories > Run > type ‘regedit’ without quotes > OK
Registry Editor will open > Expand and go to Computer\HKEY_CLASSES_ROOT\.xlsx> right click on .xlsx > click Delete > hit Yes > close Registry Editor.
2. Re-associate .xlsx types of files to open with Excel. (Right click on the .xlsx file > open with > choose default program > select ‘Exceland ensure that ‘Always use the selected program to open this kind of files’ > OK
Note: Before you make changes to a registry key or sub key, we recommend that you export, or make a backup copy, of the key or sub key. You can save the backup copy to a location you specify, such as a folder on your hard disk or a removable storage device. If you make changes that you want to undo, you can import the backup copy.
http://windows.microsoft.com/en-US/windows7/Back-up-the-registry