Try switching another computer with the problematic one. If the second system is still slow (and NOT slow in its original location, try connecting to another switch port, or running another wire (I have a 100′ length of CAT 5e for just this purpose, its been run down just about every hallway and through every door in the building) check duplex settings on the networks cards as well. Make sure you have the most recent drivers for the card as well.
I echo to check the duplex setting. This is often the cause of poor network performance on a computer. Please post the results of connecting this computer to another port to see if the problem moves with the computer. Also, check the punchdowns, maybe something happened at one end of the cable run or the other. A good continuity tester would be useful for this. Try putting a hub with a sniffer between the computer & LAN. See if there are CRC errors or other errors indicating network issues on this link.