Check what you're running and your settings(top)Various programs and settings can cause trouble. If you have a funky network setup at home, and have things bridged together or anything like that, disable it. If you have any other networks showing up, disable them too, just for the hell of it. If you have anything running DHCP, disable it.
Ethernet(top)Each LAN party is like a massive troll meetup. Make sure your Ethernet cable is plugged into a port on the nearest switch, and into the back of your PC.
Network Place(top)Network Place should be set as a "Home Network". You can check this by going toControl Panel > Network and Internet > Network and Sharing Center.If the network is not set as a "Home Network", you can change it by clicking on the indicated link.
Windows Troubleshoot(top)Might sound lame, but seriously, just try it. Right click on your network icon (the picture is a wireless icon, I'll change it sometime ), then click the troubleshoot option. Might take a few minutes, but if it comes up with any fixes, apply them, and see if that works.