Trojans and open ports

Today when i logged into my router i noticed the password was set to default.When i went to open some ports on my router,at the forwarding tab,then to upnp forwarding,i noticed that someone open some ports in the 60000 range internal port the external was in the 300 range,well i found out that is the trojan port range.I neevr open those ports,and no one that has access to my computers did,this is a fact!So i dont know how this happen.I have run 4 different virus scan and nothing has come up,ill also note that the internal ip was 192.168.1.255,when u addd .255 to the end,it opens the ports for all the computers on the network with my linksys router.My question is can someone get into my computer with these ports open in upnp forwarding.Dont they need for me to have the trojan software installed too?If i have no trojan virus on my computers,can they still use these ports to get in.