Can you describe the topology of the network you are using, and the number of computers on the network?
When you say that "other computers on the network show completely different results...", does that mean that this computer shows 2 Meg, and all the others show 10, or do they all vary?
If all the hosts show 10 meg with the exception of this computer, it's likely something physical, or an issue on the PC. You have already tried changing the cable, but what about trying a dirrent port on the switch (or router) and/or PC.
If all of the different hosts are showing varying results, then personally, I would shut down ALL traffic on the network, and run the speed tests 1 by 1. The problem with running speed tests on a network is that just because you have 10 meg internet doesn't mean every host will get 10 meg all the time. If George is sitting on a computer downloading songs from iTunes, while Johnny is playing WoW on his laptop, and Suzy is playing PS3 or Xbox over the internet, your results won't show 10 Meg. It doesn't mean that you don't have 10 meg service, just that you don't have the full 10 meg to use from that host, it's a shared medium.
Also, you haven't mentioned what type of service this is, but with many, you can log into the modem, and get vital information such as any errors it sees on the line, as well as the speed it is connected at.