I think there's a combination of possibilities on why the second system out performs the first.
here are some of my thougths -
XP's out perform even pentium 4. So naturally it will out perform P3's. I'm not sure if xsi is optimised for dual processors though.
The os may handle system resources differently. Although i think winXP is based on winNT there's bound to be some difference in the way they manage resources.
What type of ram is used in each machine? I assume your using DDR for the XP setup. But what about the P3?
The gfx card is also an important factor. Not really sure how a GF4MX compares to GF3 but it's bound to be slightly faster.
Just out of interest markman how did you bench mark your systems?
Generally i think the type of hardware is more responsible for the increase in performance and not the software.