asp.dll, aspnet_isapi.dll and aspnet_wp.exe

For example, a Web site that serves up classic ASP pages has the .asp extension mapped to the asp.dll ISAPI extension. The asp.dll ISAPI extension executes the requested ASP page and returns its generated HTML markup.

When an ASP.NET request is received (usually a file with .aspx extension),the ISAPI filter aspnet_isapi.dll takes care of it by passing the request to the actual worker process aspnet_wp.exe.

http://www.codersource.net/aspnet_compilation_execution.html
http://aspnet.4guysfromrolla.com/articles/020404-1.aspx
http://www.eggheadcafe.com/community/aspnet/17/10040175/check-this-to-download-la.aspx