Well you asked about the code so I wondered if you were using the code. What you may not understand is that all the code is compiled ahead of time, it is not the same as a scripting language where the code just exists in text files and is interpreted at runtime, like in PHP or Classic ASP where you can just edit a file to make changes if you knew how you, but in this case there is no way to make changes in the deployment package even if you did know how to change the code, there is no C# code in the deployment packages, only dlls. So to make a code change would require working with the source code in Visual Studio then re-compiling after making changes then packaging and deploying the build without the source code.
But, I digress, I do not know why it is not working for you and would be interested to hear if anyone else is having this issue on Win 2003. I ran mojoportal.com for about 2 years until recently on win2003 and the page not found module worked fine for me and no-one else ever reported issues with it so I have no idea why it is not working for you.
Best,
Joe