or put it on a USB flash drive
(http://i37.tinypic.com/2jseph.jpg)
If you have already downloaded - or plan on downloading - a purchased copy of Windows 7, you may need a tool to help you turn the bits into something bootable so you can actually start installing your new OS.
By some amazing coincidence, Microsoft has a tool designed to handle just such an emergency! Grab the Windows 7 USB/DVD download tool (http://store.microsoft.com/Help/ISO-Tool (http://store.microsoft.com/Help/ISO-Tool)) and follow the on-screen instructions and you'll have a bootable DVD or USB flash drive in no time. You'll also need your downloaded Windows .ISO file, of course. If you plan on using a flash drive, it needs to be 4Gb or larger.
Microsoft's page includes exhaustive (and I mean exhaustive) instructions and a list of frequently asked questions on the download tool page.
More experienced users may want to stick with the app I've mentioned before - WinToFlash (http://wintoflash.com/home/en/ (http://wintoflash.com/home/en/)). It's fully portable, whereas the Microsoft app has to be installed.