In order to use the installer disc that x BlueRobot linked to you will need to have an external USB DVD drive to use with your eeePC. You would probably be better off following these instructions: How to create a bootable Windows 7 USB flash drive | Ars Technica
which will lead you step-by-step in creating a USB thumb drive installer for Win 7.
As part of the install process the installer will format the drive (if needed) so you don't need to worry about that. Before you start, go to the ASUS website and download the drivers
for your eeePC for the wifi, network, video, etc... so that you can install them after installing Windows.