on Your main web server do a

telnet localhost 80
GET /

and see what's the ascii output, is it also showing "http 404" error? If it is then do a

netstat -na | grep -i listen | grep \\.80

to see what else is possibily using port 80

If GET / is not showing the "http 404" error, go to a client machine, telnet to the webserver ip address at port 80
example

telnet xxx.xxx.xxx.xxx 80
GET /

and compares the output