It could be the result of a bad cable. Generally the key causes of packet loss though are collisions at the switch. Usually that's the result of either confilcting IP's or massive bandwidth. So if you see your switch flashing like it's on speed you have collisions. If that's the case you may have a virus that's trying to send out packets of data that are or are not getting out.

You could try installing Zone Alarm and it'll monitor what's coming in and out of your system. It won't show packet loss exactly but if you see a whole bunch of shit that you don't recognize it's a good place to start. Also massive add/spy ware can cause it as well as those 'free' shit software like the weather bar, and hotbar etc..etc.. that you can add to yoru browser. Major bad mojo for packet loss.