Specify Word Doc Filename In Save Dialogue Box

I have a webform that creates a word document as it's ouput (yes I know it only works on IE).
function createWordDoc(){
Doctxt = "some text here"
var w=new ActiveXObject("Word.Application");
w.Visible=true;
w.Documents.Add();
w.Selection.Font.Name="Times New Roman";
w.Selection.ParagraphFormat.Alignment = 1;
w.Selection.TypeText(Doctxt);
w.Documents.Save();
}

My issue is with saving the file. At the moment, in the above code, it opens the save dialogue box, and populates the filename field with the first line of text in the document. What I want to be able to do is populate the filename field with a variable filename created by the function. I've tried the snippet below, which works with the dynamic filename, but it autosaves the file to "My Documents" folder without prompting. I need to be able to allow the location to be changed.
var FileName = "Myfile.doc"
w.ActiveDocument.SaveAs(FileName);