Chances are good that your users need to download files from your organization's Web site. And since providing a download is as easy as providing a link, you certainly don't need to read an article about the process, right? Well, thanks to so many Web advances, there are many reasons it might not be that easy. Maybe you want the file to be downloaded as a file rather than shown as content in the browser. Maybe you don't yet know the path to the files (or maybe they're not on disk at all), so those simple HTML links aren't possible. Maybe you need to worry about your users losing connectivity during large downloads.
In this article I'll present some solutions to these problems so your users will have a faster, error-free downloading experience. Along the way I'll discuss dynamically generated links, explain how to bypass default file behaviors, and illustrate resumable ASP.NET-driven downloads using HTTP 1.1 features.
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ref:
http://weblogs.asp.net/pleloup/archive/2006/08/25/Build-Smarter-ASP.NET-File-Downloading-Into-Your-Web-Applications.aspx