I'm not sure that this really means anything. After all, the windows version of iTunes is a completely different program, despite the fact that it looks and does the same thing, so some things may be faster, some slower. For example, I've always seen that iTunes is slower to rip CDs in OSX.
I think that the visualiser is run by the CPU, and not the GPU. If it is multi-threaded, then I'd expect it to run at half speed because XP home only lets you use two of your cores.
David