It’s possible that ASP.Net is not registered correctly on your system.
Try running aspnet_regiis from the command prompt.

Here’s the default location:

C:\WINNT\Microsoft.NET\Framework\<<ASP.Net Version#>>\aspnet_regiis.exe -i

Windows Server 2003, you must use aspnet_regiis -i -enable
This is because of the “Web Service Extensions” feature in IIS 6

(if you install VS.NET or the framework without IIS installed, and then go back in and install IIS afterwards, you have to re-register so that ASP.NET ‘hooks’ into IIS properly.”