Well, it's good to know that you've got a multi-boot setup which rules out the hardware. Good first start.

Have you run any programs which modified the internet connection settings? A common one is a "tweak" utility which is meant to improve download speeds and ping times in Quake games. These modify things like the MaxMTU value in the system registry. These utilites are meant for modem connections only, and will totally screw up LAN connections.