If you never have run a gnutella client on port 80 it may be some kind of attempt to use the Gnutella network as a tool to launch a DDoS attack but I still have my doubts.

LimeWire at least will never request a file twice from a host that has sent a 404, nor will LimeWire send the address of a host as alternate location unless it has successfully tried that host (and got a 200).

There seems to be at least one non-LimeWire host (or a modified LimeWire host) that is apparently still sending alternate locations for your http server. Are there other computers using the same address? You might want to check if someone else is using the same IP and propagates port 80 as listening port for some reason (some people seem to do that because they believe they might get around firewalls that way).