I found this on FileSoup:
By default Windows operating system uses 20% of your bandwidth to remove this.
1) Go to RUN and type gpedit.msc it will open a dialog box.
2) Go to Local computer policy> computer configuration>administrative
templates>network> Qos Packet Scheduler.
3) On the right hand side you will find Limit Reservable Bandwith open it.
4) Enable it and set the percentage to zero.
5) And u have your 100% bandwidth.