As the name implies, it is the system, ie the kernel, so you can not stop or kill it without killing all of windows. Moreover, the System process is like
svchost, that is, it contains multiple items, so you would need to figure out which specific component is the one occupying port 80. The concern is that most of the components that run in the System component tend to be drivers and the like, and as such, have high-level access to the system. A webserver does not need such access, so I would be extremely wary because it sounds like a virus or rootkit is running on the system and is providing a web interface.
You will want to run some security software to check your system for malware.
(From what I can tell, it seems that for some reason, IIS does implant itself in the System instead of a dedicated process like a well behaved program, but if you uninstalled it, then it cannot be the source of the problem. That said, check
Services.msc to make sure that IIS is not still present and/or running.)
To find the specific component that is occupying port 80, run Process Explorer and double-click System in the process tree. Look in the Threads tab and sort by Start Address. Now look through the list of drivers and see if any look suspicious. There will be some Windows files as well as driver files for your video, audio, network cards, and such, but you can Google any that do not look familiar. You can check TCPView to monitor any connections to your system.