I once caused myself (embarassing) trouble by re-installing Windows. All of a sudden I had no internet connection. It turned out the problem was simply that I had forgotten to install into windows the drivers for my mobo that came with it on the CD. Remember, the motherboard itself contains many devices (like LAN ports) that require drivers installed in Windows so they can be used. So take shovenose's advice - check if your LAN port driver is installed.