Life Hacker has debunked a bunch of Windows performance tweaking myths, you can check it out over here. Here's one of the myths:
Disabling QoS to Free Up 20% of Bandwidth
This tip made the rounds with people believing that Microsoft always allocates 20% of your bandwidth for Windows Update. According to the instructions, you were supposed to disable QoS in order to free up bandwidth. Unfortunately this tip was not only wrong, but disabling QoS will cause problems with applications that rely on it, like some streaming media or VoIP applications.