for some reason the association is wrong. right click the excel file and select properties. the window that pops up will say what program it opens in. next to this is a button that says "change" click this. it should list excel as a recommended program or show it, if not select browse and and navigate to excel.exe and select it. normally they are under program files/microsoft office/office 12
this will change the association back to excel and open it right from here on.