reply to Katikiany
Re: 1 PC on network very slow Yep start at the bottom and work your way up.
Any time I have data transfer issues I check the cable first.
I have seen cables worn through by vibration, chewed on by rodents, nicked and cut by sharp corners or metal in drop ceilings.
Next check the nic settings its easy to forget to double check that a nic is to auto. (You had already done this.)
Story: I was working at a hospital and the staff on one floor in an annex building were complaining that everything was always slow and noone had ever been able to fix it. Since I doing a rollout and scripts and installs were coming down over the network (and damn it WAS slow). I told the floor manager I would look into it. Turns out the problem should have been fixed two years before when the switch had been replaced with a gig-e switch with a fiber connection to to main building. I went back to the machines we were redoing and found that they were still hard set to 10mbit nonduplex.
After redoing the driver and switching them all to auto suddenly everyone was running at full speed. I told the IT manager I reported to the cause and the solution. He was NOT happy and the admin guy in charge of the area got a little chat.
Next I check to see if it is the port on the router or switch.
I have found that if you are doing a star burst arrangement the switches chained of the first switch should use the lower port numbers on a switch especially if it is an older one. And always using port number one as your uplink port unless you have the manual or know for sure that some other port can be used as an uplink.
Finally checking your router itself, modem, etc. depending on your layout.
In some cases checking the server but I usually leave that up to the server guy as it is usually not me.