With VB 2005, get into into the current, already open PowerPoint (Office 2003) presentation (and, for example get its name):

Dim ppapp As New PowerPoint.Application
MsgBox(ppapp.ActiveWindow.Caption)

It works. Then, why the same with Excel:

Dim xlapp As New Excel.Application
MsgBox(xlapp.ActiveWindow.Caption)

produces error "Object reference not set to an instance of an object"?
It bewilders me. How can I get into the Excel sheet that is already open?

“Why so?” is not much of a subject line. It is assumed from the fact that you are posting a question t all that you have a question...

I would think that the new application object has no active window, because no workbook has been opened.

Try stopping the code after the line Dim xlapp . . ., and examine that variable in the watch window to see if the object variable itself is not yet anything, or if it is the ActiveWindow property that is nothing yet.