See
http://support.microsoft.com/kb/288902
Check out:
set xlApp = GetObject(, "Excel.Application")
If an instance of Microsoft Excel is running when this code is
executed, you have access to the running instance's object model
through the xlApp variable. If no instance is running, you receive the
following trappable run-time error message:
Run-time error '429':
ActiveX component can't create object
Then you can use xlApp.Quit to close it.
HTH,
JP