It depends. I'm assuming that the drive you want to format has windows on it. In that case you will lose all data, unless the drive is partitioned and you format a partition that doesn't have windows on it, or you are formatting a separate drive that doesn't have windows on it.
For a diskless install of windows, if you have a big enough flash drive you can use this tool:
http://www.microsoftstore.com/store/msstore/html/pbPage.Help_Win7_usbdvd_dwnTool