You need to fix the installation of Excel because it seems it got corrupted or one of its main file is not in a proper condition. To do this, go to Start > Control Panel and look for either Programs and Features or Add or Remove Programs. Locate Microsoft Office on the list and click Uninstall/Remove. Instead of uninstalling, select Repair and follow the instructions. You might need to restart the PC at the end of the repair operation. When you come back to the desktop, the application should work just fine.