What would cause unbelievably slow network connection??

[MustISO]

I just built a new PC so my old one is going to be used as a standby. The machine has worked flawlessly for two years. I just installed XP fresh, and when I try to get the Windows updates or AV updates the network/connection is incredibly slow. Like 12KB/s on an 8Mbit cable line. None of the other machines have this issue.
I've tried:
another fresh install
updated NIC drivers
a new CAT5 cable
different port on the router

I've never seen anything like this before.