Are you talking wireless?? What are you using for hardware, router/gateway etc. A bit more info on the hook up(s), proximity to system(s) might help.
I have a few PCs on a broad band connection through comcast. Recently my vista os one after trying to update to sp2 and loading diablo 3 and removing norton anti virus this pc has massive degrade (to 200 kbs) of speed. My windows 7 laptop on the other hand maintains 2.5 million to 10.5 million bps.