Computer keeps restarting

Hi,

I have a computer at home that is on all of the time. Every four hours it will restart on its own. Can anyone tell me what could be causing this to happen please? i am running Windows 2000 with Office XP. Running Kazaa all of the time. Sometimes when it reboots, you get the BSOD(blue screen of death!) with reference to the internal modem.
Hope I have provided enough information!
Look forward to a response

Mil