If it's reserving a portion of your bandwidth, perhaps it's a QOS type of issue?? I guess? I don't know what I'm talking about, but I had that problem once.
Maybe grab TCP Optimizer from here, run as admin and see if it can help
http://www.speedguide.net/downloads.php
Since you already tried different cards and versions of drivers that's all I can think of.
PS:
http://www.reginout.com/Is-your-Wind...oing-Slow.html