I'm not sure on that one. can be network traffic at the site you are downloading. Things you have to remember is the internet is not a straight forward architecture. To access your friends computer from next door, you might shoot to japan, egypt, california, new york, canada, belize, jamaica, australia, then next door. It can also be that windows sux. Now I have a question. Why is it my 54mbps wireless connection drop down to 1mbps when I'm streaming audio/video or p2p? I know it is not the distance from the router because it is only three feet from me and those activities are the only time it does that. Downloads and uploads are fine. It also disconnects my desktop (which is 1.5 feet from the router) any time a p2p is running, whether it is saving to that comp or not, just faster if it is. That's wierd.
BTW, visit
http://www.dslreports.com , test your connection, test your settings (all under the tools link), and then tweak using dr. tcp and follow the FAQ. Should help solve a few problems.