You use the FindWindow function to find it by knowing it's caption.

Code:
Private Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" (ByVal lpClassName As Any, ByVal lpWindowName As Any) As Long

Dim h
h = FindWindow(vbNullString, "Calculator")
If h = 0 Then
MsgBox "Window was not found!", 16
Else
MsgBox "Found!": Exit Sub
End If