Ethernet Slow 1 PC Only (Driving Me Crazy!!)

I have a machine where my connection speeds are extremely slow. I bypassed all my routers, switches etc.. and plugged directly into my cable modem to test and make sure the problem has nothing to do with other hardware and it's confirmed. If I plug any of my other machines in they all work fine, it's purely isolated to this one machine.

When I run a bandwidth test using speakeasy.net or speedtest.net the it acts totally crazy and the needle jumps all over the place and finishes in seconds, somethings jumping all the way to the max and giving outlandish readings of 177mbps download speeds and such. I took the machine to a different location to double confirm, ran the speedtest and it did exactly the same thing. So I'm 100% positive it has something to do with the machine.

Now after that I bought a new gigabyte card, disabled the onboard ethernet and am now using the card. But still no resolution, same problem. So I'm assuming now that it has to be software related or some other hardware associated but I'm drawing a blank as to what else.

FYI this machine has been running at the same location for more than a year and this problem seemed to just surface out of the blue. It was working perfectly fine before. I tried changing the ethernet card settings from auto to 100/duplex, half etc.. and still luck. If anyone has any suggestions it would be greatly appreciated, I'm about out of ideas. Thanks

OS = Windows XP sp3
CPU = Intel Core2Duo E8400 @ 3.00Ghz
RAM = 2GB
Motherboard = AsRock G31M-S