Microsoft reserves 20% of your available bandwidth for their own purposes like Windows Updates and interrogating your PC etc

You can get it back:

Making a bootable USB flash drive for Windows Vista and Windows 7 isn’t all that tricky, but it’s always nice to find an app that simplifies things. Not only does WinToFlash make the process about as easy as it can get, but it can also create Windows XP, Server 2003, and Server 2008 installers.