Have you created a Virtual Directory for your app?

If you dont know what I mean, what you need to do is open IIS. Righht click on Default Web Site, choose New, then Virtual Directory. Follow the steps from there. It will ask you what directory to point to. then when you run the site, access it using your servername/virtualdirectoyname

for example

http://localhost/myvd

Perhaps you just stuck you .aspx file into just the inetpub/wwwroot directory, in which case I dont asp .net will work like that like old asp used to.