Last I checked, you can't end the "system" process, and if you do, I'm guessing it's going to have catastrophic effects. I'm not going to try it on the PC I'm on right now either!
It would seem that something inside Windows itself is listening on :80 - I'm going to guess that it could be something malicious. The best way to find out is to either:
a) Open a web browser to localhost and see what comes up
b) Start Telnet and telnet to localhost 80 and run some basic HTTP GET (e.g. GET /) and see what it returns
B is the better option if you think that you might be hosting malware, as you don't really want to infect yourself again. Although maybe it won't matter.