Modem Connection

I am having an issue with my home pc (XP Professional) which keeps dropping its connection to the internet, I have checked all the usual places (modem properties, internet settings)and have even changed the phone line but to no avail, also when the connection drops it will not reconnect me as it will not open a port, I have to reboot the computer to reconnect. To compound the problem the other computer in the house which uses the same ISP does not have this problem, making me think that it is asetting within XP or hardware, please help it is driving me mad