You have nothing to worry about anyway, this is harmless gnutella traffic. You've said this occured before you installed a gnutella client, understood. But due to DHCP your IP address changes on a fairly regular basis (depending on your connection) and you simply aquired the IP address of someone who had recently been on the gnutella network. No big deal.

BTW, a ping shouldn't scare or **** off anyone, it doesn't do anything. And "your firewall caught it so you're okay" is bullshit, you're okay anyway because you don't have a service listening for traffic on that port (even if you did, it'd most likely be a gnutella client which don't currently have holes to exploit). Noone can just aim a sharply pointed packet at your computer and "hack" it. You must be running some form of server (web, email, ftp, gnutella servent, etc) to receive and process the traffic coming in.

If you're not (*Remote access trojans run as servers, so check for those*) then you don't really have much to worry about (at least not on that front.)