If you are trying to run a .NET app (aspx page) under Vista IIS 7 and you encounter the following error,

HTTP Error 404.3 - Not Found

here's what you do to circumvent

Go to:        Control Panel >> Programs and Features
Click on:      Turn Windows features on or off (LHS)
Check the boxes shown below

Thanks to:

http://www.netomatix.com/Development/IIS7_404Error.aspx