ASP/ASPX pages giving 404 errors

In case I need to do this in future... I have been trying to copy a web site to a new web server (Windows Server 2003) and getting very confused about why .ASP and .ASPX pages were giving 404 errors, but HTML pages were displaying correctly. The reason was "ASP.NET" and "Active Server Pages" in the Web Service Extensions directory were both set to "Prohibited".