I would start with drivers. Find out what adapter you have and obtain the latest driver from the manufacturer's website (by this, I mean go to the chip manufacturer, not the system OEM - i.e. get Intel network adapters from Intel's website, not from HP if your system was assembled by HP). Go into the Device Manager and find your wired lan adapter. Right click the entry and select Uninstall, and check the box to delete the existing driver as well. Reboot once the driver is removed.
Now go find the driver you downloaded earlier and install it. Reboot again for good measure. Check to see whether your downloads behave the same way and post back.