Whenever I browse to one of my ASP.NET web pages, Internet Explorer keeps on asking me if I want to download it, or it just displays the ASPX source. Why?
IIS does not have the correct mapping for the ASPX file extension. This can happen if you install IIS after installing the .NET Framework, or if you uninstalled and reinstalled IIS.
To repair IIS mappings for ASP.NET, you can run the Aspnet_regiis.exe utility.
For more information check out KB306005