fuser should be run as root.
If you did not get anything back from the command,
then fuser could find any process ocupying port 80.
As another test try:
telnet localhost 80
You should get a "connection refused"
If you can connect, and a cursor sits blinking and waiting in your terminal,
you might start to worry, because some proccess is running a listening socket on port 80, and root is not able to find out what PID it has.
(Might be a rootkit, worm or someting like that)
If you get a connection refused, "everything" is OK, and you should look further into the webserver config.
What webserver are you trying to use, what configuration ?