The simplest way is to export the file as a csv (comma seperated values) with a .xls extension which will open in excel although it's technically not an excel document, the alternative is to use the office interop for excel, only trouble with that is that office will need to be installed on the web server plus license. There are some components which do this kind of export e.g. aspose excel but obviously you would need to pay for it. Obviously in both cases the client would be prompted to save the file to their disk as the documents will be streamed from the webserver, having the application automatically save the document on the clients machine would be a lot more complex.

Heres a good article about the csv method:
http://www.dotnetspider.com/kb/Article963.aspx