
press F12 or whichever key it is on your system to boot to you USB device when you want to install the OS. Maybe you have some trouble in your BIOS configuration as well? Check the boot order, you want your harddisk to be first there. Wouldnt want you to loose important documents, family photos etc. However, you need to make sure you have a complete backup of whatever data you have on your old installation first.

What happens when you try to install Windows 7 from a USB memory stick? Guides on how to make a USB memory stick into a bootable Windows 7 installation medium: or Personally I perfer a clean install whenever I want to install a new operating system, which means I would format the system partition.
