Installing an OS usually requires a bootable CD/DVD. With Windows 7 you can skip using a CD/DVD for installing and instead make use of a USB drive, this is particularly helpful for people who use netbooks which do not have a optical drive.
If you are looking for a easy way to copy over the Windows installation files to a USB drive a handy freeware called WinToFlash can come in pretty handy.
WinToFlash provides users with a wizard that will import the contents of a windows installation CD and copy over the files to a USB drive making it a bootable replacement for a optical drive. This application cal also be used to create a bootable USB drive from a LiveCD.
WinToFlash makes creating a bootable USB drive a piece of cake.