Microsoft offers tool to burn Windows 7 ISO - or put it on a USB flash drive
By some amazing coincidence, Microsoft has a tool designed to handle just such an emergency! Grab the Windows 7 USB/DVD download 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. It's fully portable, whereas the Microsoft app has to be installed.