To set the file association of an excel (.xls) file to open with your Excel program, simply right click the file and select Open with, followed by selecting Excel as the associated program. Don't forget to Apply/Save your settings. This will associate those file types with Excel and should open them properly in the future.
One other thing you may want to do is to visit the Office Update site located at:
http://office.microsoft.com/en-us/downloads/maincatalog.aspxYou'll need to validate your copy of Microsoft Office...then continue to follow the directions in order to update your version of Office.
Make sure you have your Office software available...just in case you need it.
Continue to visit the Office Update site to make sure you have all of the updates installed for your version of Office.
Once this is done, you may continue to see this Installation wizard...it's Excel's way of completing an installation if there have been any changes made to its original configuration.
Hope this helps...and please, keep us posted...