Don’t use the same USB drive you plan to install Windows with. Either connect to a network or plug in a USB drive and copy everything want to keep. The first thing you will want to do is make sure you get everything you want off the computer.
Maybe your Netbook came with Windows XP and you want 7 on it or vice versa.
There are a number of use case scenarios where you would need to install Windows onto a computer without an optical drive. This can save you the cost of buying an external optical drive for your Netbook. But it is not impossible and with the right tools, it is doable. If you have a notebook computer without an optical drive and want to install Windows onto it, you have a challenge.