Vista has some utilities that might have helped but on XP I really do not know.
I think download bandwidth for most Broadband deals is usually much more than upload bandwidth. I think the PC that has the hgh level of receiving activity may be in use as a Bot (stealth), or someone else used that PC for a period of time (heavy downloads) and the main user was not aware of it.
Also the PC "dragging" suggests a possible Virus as well. Might simply need a defrag though.
Beyond that it could be a number of things but its not my area of expertise.
Have a look at
this article it might help.