2 answers are sought here I expect, so....
Creating a registration system for a web site minimally would involve storing registration data. So I'm afraid you'll be needing to do a little more than simply cut some HTML to achieve this. And it makes a difference if your web server is Windows, Mac or Linux - if you're using Windows IIS as your web server, it's pretty straightforward to create a short script that will dump registrant data to a text file. If this registration info is going to be used by other applications, you might want to consider involving a database like Microsoft Access or SQL Express or perhaps even a full-blown instance of SQL Server, depending on what you want to do with your data. In all cases, the work involved in storing the registration data itself would be minimal.
A comments/discussion threads application would also need to invovle more than just plain HTML - because, again, data is being stored and in this case retrieved to be displayed on the web site again afterward. Again, using Windows as the example, involving SQL Experess or SQL Server with the IIS web server would be the most logical approach. However, I would point out there are lots of discussion/comment/thread examples available just by searching online and were I doing it I'd be inclined to adapt one of these examples to work with my own custom application. (No need to re-invent the wheel I always say.)
Hope that helps a little....