I just spent the last hour troubleshooting an issue with an upgrade. It had to do with upgrading a website to a new version using .net 4 on an iis 6 (server 2003) machine. I was consistently getting a 404 error when I switched to using the 4.0 framework
I had reapplied 4.0 framework to iis using aspnet_regiis. Even created a new app pool.
In the end it turned out to be a very simple miss on my part. To make it work I only had to enable the “web service extensions”. The ASP.net v4.0 was prohibited. I had completely forgot this even existed. IIS 7 does not have this design model. It is amazing how simple things can cause a lot of challenges.