Some blog posts around the net and some computer tweaking applications make believe that Microsoft always allocates 20% of your bandwidth for Windows Updates. According to the instructions or settings in those software, you were supposed to disable QoS in order to free up bandwidth. Unfortunately this tip was not only wrong, but disabling QoS might cause problems with applications that rely on it, like some streaming media or VoIP applications.
Click here to read the official Microsoft response: “There have been claims in various published technical articles and newsgroup postings that Windows XP always reserves 20 percent of the available bandwidth for QoS. These claims are incorrect… One hundred percent of the network bandwidth is available to be shared by all programs unless a program specifically requests priority bandwidth.”
I hope this helps to clear out the situation and the actual use of QoS.