Protect the ARP cache?

The ARP cache keeps track of the MAC addresses of computers you communicate with and what IP address they tie too. On a PC, the only entries that are likely to be there are ones for your gateway and any network printers you have.
What an ARP poisoning attack is usually looking to do is make your computer think a different machine is your gateway than the router you set up. That way, all your traffic (e-mail, banking sites, etc) goes through it before being passed out to the internet. It allows someone to set up a "man-in-the-middle" attack much more simply and to sniff all the information going in and out from your network.