Hmm, I wonder if it is your NIC instead of the Router. Here is a few Loco ideas to try:

Plug in the Cat-5 to the NIC but make sure the other end is not plugged-in to the Router. Is it still slow, then likely the problem is could be with the NIC or its drivers. I would uninstall the drivers, then remove the card (if applicable) then install another NIC to see if it made a difference.
If it is still fast that way then your LAN settings or the Router could be the culprit indeed, try another port on the router or check the router settings.

~TL