Trying to run Asp.Net code from a Windows 2003 server running IIS 6.0 and I got a lovely "The page cannot be found"? 

This is bizarre as I have installed .Net 4.0 on the server, correctly ran C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe (just encase) and abortion at 6 weeks also abortion information correctly changed the properties of my Web Application to ASP.Net Version 4.0.30319

Looking at the actually Web Page error and I notice 404.2 - Lockdown policy prevents this request.  Ok nice so you can install .Net 4 for Asp.Net code but by default not actually use it?

Next check what extensions are available on the server:- open command line and enter:-
c:\Windows\System32\cscript iisext.vbs /listfile

Notice that C:\Windows\Microsoft.Net\Framework\v4.0.30319\aspnet_isapi.dll has a Zero meaning disabled.

OK to enable just run the following:-
c:\Windows\System32\cscript iisext.vbs /enfile C:\Windows\Microsoft.Net\Framework\v4.0.30319\AspNet_Isapi.dll

And bingo all code working fine now.