Caveman, it may not be anyone trying to "get in" to your LAN.
You may have gotten an IP from a Kazaa (or any file sharing program) user and you are getting hit by the other Kazaa users. This usually will subside in about 24 to 48 hours. This is probably the most likely due to the number of these users on the Internet. Bandwidth hogs.
Or, there could be other users on your subnet who are running with file and printer sharing and your LAN is being hit by all the various probes MS sends out to the subnet when announcing itself and searching for the other nodes on the subnet. This will be sporadic and mostly effects cable networks, due to the network architecture.
Or, someone on your subnet is running a program that has to do a broadcast and you are getting hit.
SPAMMERS hit port 25 of every IP address on the Internet, continually, so this could be it. They are just looking for open relays. Sick lot.
If your firewall is doing any logging, then you have the IP address. You can go to
http://www.arin.net and find which ISP owns that IP address.