After setting up IIS6 on my Windows 2003 Server I tried to run an ASP.NET application. I was getting a 404 even though the aspx file existed. If I hit any non ASP.NET extension (txt, xml, html, htm, asp) it worked fine. I remapped the aspx files (Knowledge Base Article) with no luck. After removing IIS and then reinstalling, making sure that I included ASP.NET in the IIS install it worked.