Help Please !

[snoogens]

I just got a asus P4C800 motherboard, with a P4 3.0ghz cpu, for some reason when i connect using my dial up modem, it automatically cuts off after about 4 mins,also my onboard lan isnt being recognised by XP either, ive check my phone lines, my isp, the modem works perfectly on all other PC's and i've got the latest version of the bios but still no difference, the modem still automatically disconnects and my os still isnt recognising my lan, ive checked some other forums and have found other people with the exact same problems but with no awnsers on how to fix them, i'd really appreciate it if someone could help me