Network goes down after a few hours

I have Windows XP Pro installed on a custom built computer and have been getting this problem for a couple days now. I leave the computer on all the time to access my online school, but recently the network connection just goes dead. I will come back to the computer after a couple hours of inactivity and no network connection. I can’t see my router or other networked computers. No IP address or any network information is in the status section of my network controller. I have an MSI AMD Nvidia Nforce 4 motherboard with on board ethernet. Thank you in advance for any help.