it's brand new. works fine with my other cable. but i can't hook up this other pc with the blue cable.
this is a known problem with this particular cable, and others like it. i just don't know what the explanation is.
Ok, this is a long shot, but it happened to me about 4 months ago. Each RJ45 male end has 8 contacts and each contact is separated by plastic ridges. When you plug the RJ45 into the female plug (ok the socket end for you gender-phobic) the 8 wire "fingers" need to drop between the plastic ridges in order to contact the metal on the RJ45. (see the attached picture)
In some Asante switches I was using, the 8 metal "fingers" in the RJ45 sockets were just a little bit too wide for the clearance in the male plug. Some of the fingers just couldn't quite drop into the slots. After wiggling (is that a technical term?) it for a bit, they would work themselves into the slots and everything would be ok. I suspect either the finger dimensions were out of spec or the slots on the RJ45 ends were out of spec, but to this day I don't know which.
Bring it down to your local mom&pop computer store.. see if they have a cable tester, and see if they can test the cable.
are you sure it's a straight cable?
here's a quick test for a cross-cable: connect one end to port 3 of your switch, and the other end to port 4 of your switch(or any ports). If they show link lights, it's a crossover (note: do not leave this plugged in for a long time, as it will broadcast-storm your network)
is it cat 5e? because in some cases the cat 5e is shipped with rj45e connectors, meant for Cisco routers, work on some and not on the others.Huckleberry has explained it above, same problem as the rj45e's
Then the cable is a crossover.
Modems are created to be plugged directly into a computer. To plug them into another networking device like a hub,switch, or router, you need to use a special kind of cable called a crossover. You cannot connect regular computers to a switch using a crossover.