I want a basic backup of my system as a fresh install of windows 7
Well, which do you want? A fresh install, or a backup up copy?
Like brett said, you just can't copy from on PC to another, and expect it to work. There are driver issues, etc.
And how would you copy from a USB drive without the OS already installed?
Plus, it is a violation of the Microsoft EULA.
If you are going to build a new PC, buy a new Windows 7 disk, too.
Copy all your "important files and data" to the USB, and then to the new rig.