Can you give any possible technical explanation for how the operating system on a given PC could affect the speed of data from the Internet to the network interface?
Sure: Windows has a lot of internal "code fat," in addition to having to run necessary applications like antivirus, that take up CPU/system resources and slow down the broadband conneciton.
I couldn't compare XP on that system because it won't even run on a lowly K-7.
Windows spyware phoning home. Botnet trojan participating in a DDOS. More innocent is simply the design of the network stack. OS X uses the time-proven BSD stack, and Linux's stack is also good, but Windows XP's simply sucks, an attempt to fit a limited version of the BSD stack into the quite different Windows API (which is why Vista has a completely new networking stack).