Unable to renew IP address

I am trying to hook up a second pc to my Marconi combi Premium modem. When I do a clean install of Windows XP SP2 I get the connection and can surf the net. Can reboot pc and connection is still avaliable. Next day it says limited or no connection and I cannot get it to connect again unless I do another claen install of XP. What can I try to resolve this problem?Have tried all the ipconfig commands , nothing helps. have also tried disable / enable , it only comes back to say no can do. My other pc is running fine with the exact setup as this new one.