J Lunis was telling us:
J Lunis nous racontait que :

Why are you setting a folder as a file?
Where is the Dim statement for "filename"?

It depends what you want to do.
You can use GetObject or CreateObject, or even the New keyword.

Normally, we test to see if the application is already running, in which
case we use GetObject. If there is no need to test we can use CreateObject.

In the VBA editor, type GetObject, select it and hit F1 for the help topic.
You could do the same for CreateObject to see the difference.

In your original code, you were setting a variable as an Excel application,
then the same variable was reset as an Excel workbook.
   Set filename = GetObject(, "Excel.Application")
   '(If Excel is already running, this gets a handle on it, if not, it
generates an error.)
   Set filename = ("C:\Microsoft Office\Templates")
   '(This reassign filename to a folder? You wrote that this worked?)
Even if you closed that last variable, the first one that was created was
still running invisibly in the background.
   filename.Quit SaveChanges:=DoNotSaveChanges

For more on this, see
   http://word.mvps.org/faqs/interdev/controlxlfromword.htm
And,
   http://word.mvps.org/faqs/interdev/EarlyvsLateBinding.htm

Another possibility: Excel is *very* sensitive to errors when called
remotely like this. There are some code mistakes that do not throw an error,
but which cause Excel to sulk and refuse to close. Most common (in my
experience) is a use of the Range keyword, eg as an argument to the sort
function, such as you would get if you copied some code from Excel VBA to
Word VBA: Both Excel and Word have Range objects, but obviously not
interchangeable. If you pass an Word range to an Excel function, strange
things happen.