I may have a fix
There is a # of things that could be wrong.
one thing i no is that you are getting some sort of siginal because iff you wernt it would say that it is unplugged. try unplugging it and see if it does this. if it does we are off to a good start. if no the problem resides in you computer box. you may have a bad or broken eithernet cable(there are 8 wires in each cable that can break) try useing the other cable that works. you may also want to put the cable (of the one that doesn't wok) in the port of the one that does work. also try going to start > run > cmd this should bring up a prompt on a black screen. type in "ipconfig renew" with the space and without the " if it has some sortof error then type "ipconfig release" then ipconfig renew se if this solves th problem, if no, reply to this and i will further troubleshoot.