Difference in speeds between XP and Linux

Hi,
I have a multi-boot system (A couple Linux and windows versions)
I just got the new U-verse internet from At&t which offers up to 12 Mbps.
My computer was in windows XP.I wanted to test the bandwidth so I went to speedtest and 2wire and both showed a max of 5 Mbps. I got angry at At&t for a minute but I had some programming work to do, so I restarted and went into Linux. From Ubuntu I decided to try speedtest and it showed 11.5 Mbps.
I restarted and went into XP again, still had 5 Mbps.
Anyone have any idea why this is and what I can do to get max speed in a windows XP enviroment?
Thank Nick