The default system behavior is that all 100% bandwidth is available, however, if there is a running application that indicates to the OS it needs to send high priority/real time data, then as long as it has the socket open, Windows XP will restrict "best effort" traffic to 80% of the bandwidth so that high priority traffic can be accommodated. Basically, applications can make this request to the operating system for QoS support using the QoS application programming interfaces (APIs) in Windows and this only applies if a specific app is requesting QoS.
If you'd like to change how much bandwidth is reserved for QoS (the default is 20% of the total bandwidth), do the following:
1. Make sure you're logged in as "Administrator" (not just any account with admin privileges).
2. Navigate to START>Run and type: gpedit.msc
3. Navigate to Local Computer Policy > Administrative Templates > Network > QOS Packet Scheduler
4. In the right window, double-click the limit reservable bandwidth setting
5. On the setting tab, check the enabled setting.
6. Where it says "Bandwidth limit %", change it to read 0 (or whatever percentage you want to reserve for high priority QoS data)
7. Click OK, close gpedit.msc
Under START > My Computer > My Network Connections > View Network Connections, right-click on your connection and under Properties (where it lists your protocols), make sure QOS Packet Scheduler is enabled.
Alternate Method (Windows Registry)
Instead of editing the group policy, one can edit the Windows Registry directly:
NonBestEffortLimit=0 (DWORD, not present in the registry by default. Recommended: 0 , possible values between 0 and 100) - indicates the percentage value of reserved bandwidth for QoS applications. Set to 0 to disable.
Notes: This tweak applies only to Windows versions that have Qos Packet Scheduler enabled.
You need to reboot for changes to take effect.
The Windows Vista registry location is the same as Windows XP
To read more about QoS and ToS/Diffserv, check the TCP Optimizer documentation, or refer to Technet/MSDN.
Technet: QoS Tools and Settings