hi
i used broad band internet connection with DLink GLB 502T modem and Dlink Ethernet card , my connection to internet is very nice but after some times its continously popup network cable unplugged hat is the reason
i used windows 2000 operating system
straight cableis from modem to ethernat card
The only thing I can think of at the moment is the IP lease time. Check the router setup and look for the lease time. Set it to 0, which is 24 hour lease. Any other number is the actual hours for which the IP is valid.Also:
http://compnetworking.about.com/od/windowsnetworking/f/cableunplugged.htm

    *  DISABLE the Ethernet network adapter if you are not using it. This applies, for example, when running a WiFi home network with computers that have built-in Ethernet adapters. To disable the adapter, double-click the small Network Cable Unplugged error window and choose the Disable option.

    * Check both ends of the Ethernet cable CONNECTED to the adapter to ENSURE they are not loose.

    * Replace the Ethernet cable with a different one to verify the cable is not damaged.

    * Update the network adapter driver software from the manufacturer's Web site.

    * Change the Link Speed and Duplex settings (using Device Manager) to use "100 Mbps Full Duplex" or "10 Mbps Full Duplex" INSTEAD of Auto Detect.

    * Replace the Ethernet network adapter if it is a removable PCI or PCMCIA card.
      First remove and re-insert the existing adapter hardware to verify the card is connected properly. If necessary, also replace it with a different card.

    * The device your Ethernet adapter is connected to, such as a broadband modem or network router may be malfunctioning. Troubleshoot these devices as needed.