One simple way would be to write a Windows script that 'faked' a DHCP renew every 12 hours. I'm not much of a Windows programmer but I'd think it wouldn't be too difficult.

That way, you could stay static and still fool the ISP into thinking you were DHCP. An additional refinement would be to plug the addresses it got back into the registry so if there was a change your static settings would update with it.