Installing Windows normally requires a setup disc, which is just fine, unless your optical drive isn’t working. Or your system doesn’t have one at all. Or the disc is scratched and unreadable. Or you really don’t have the patience for those painfully slow data transfer rates.
One effective solution is to transfer your Windows setup files to a bootable USB key. Novicorp WinToFlash has being doing just that since 2009, and despite development stalling for some time, a recent rush of major new releases means the package is finally out of beta.