Windows allots 20 % of internets bandwidth by default for various services like windows updates ,spyware checks ...this demo will tell you how u can get hold on the unused bandwidth....
First go to start then click on run.
Type- gpedit.msc
Go to ADMINISTRATIVE TEMPLATES -
NETWORKS - QOS PACKET SCHEDULER.
Click on reservable bandwidth and check the ENABLED box and change the bandwidth limit from 20% to 0 %..
And you are set to use your full bandwidth for downloading or uploading..