Most people seem to be fairly confident in routers with NAT, but just how hard is it to get into a home users machine if it is guarded by one?

I have a Linksys 4 port router (latest firmware updates) as well as a software firewall, but I can't help but be paranoid about someone penetrating my router. Is this possible without a Trojan on the machine, and if so, how difficult would it be for an attacker of any skill set to accomplish this?