Generally Excel.Exe will be located in C:\Program Files (x86)\Microsoft Office\Office15 in case of Excel 2013 / Windows 7. During installation, if another path is chosen, Excel.Exe's location changes. Default Path is also dependent upon Office and Windows combinations. Sometimes, it may not be locatable at all.
To find the location of Excel.Exe -
1. Start Excel
2. Right click on Windows Task Bar > Start Task Manager > Processes tab (Task Manager can also be opened by pressing CTRL+SHIFT+ESC)