The machine is dual-core, and was quiescent at the time. XP's scheduler is hopefully good enough to just leave the process running on one core.
It's not. Go into the task manager (accessable via Ctrl+Alt+Del by default) and change the process' affinity to the second core. In my experience, running on the second core (in both 2k and XP) tends to produce slightly faster results. Linux tends to keep processes on a single core for a few seconds at a time. - Josiah