Your best bet is to present the file for download. You could redirect to a page with a link to the download. But I am pretty sure that you can't use a Save File dialogue, because you can't create client side popups with server side code. But if you present it as a download, the client will get the browser's save file dialog.
InsertAlias is correct, you cannot call the "SaveAs" dialog box in ASP.NET.
However you can just use an old-fashioned hyperlink that points to the file. Since Excel files can't be rendered in the browser, the browser itself will open a "SaveAs" dialog box to allow the user to download the file.
Make sure that the folder that contains the Excel file is on the web server or else this will not work.
-Frinny