If you buy a copy of Windows 7 online, from Microsoft, then at the end of the process they'll allow you to download an ISO disc image. Normally you would then use your disc mastering software to burn it to DVD, but what if you want to install it on a netbook, or some other system that doesn't have a DVD drive? Then you'll probably need a copy of Microsoft's Windows 7 USB/DVD Download Tool.
Despite the name, this straightforward utility has nothing to do with downloading Windows 7: you'll still do that from within your browser. When that's finished, though, the Windows 7 USB/DVD Download Tool will create a bootable USB Flash drive, based on your Windows 7 image, that can be used to install the new operating system. It's all very simple, just a four step wizard (and if you'd prefer to create a DVD then it can do that, too).
What the program won't do, unfortunately, is work with modified ISO files. So if you've slipstreamed some additional drivers, for instance, then it'll complain about the changes and refuse to go further.
And of course there's no guarantee that your computer will boot from the USB flash drive right away: that's all very dependent on your BIOS settings. If it boots from the hard drive as normal, fire up your BIOS setup program (press a key like Del or F2 - a boot prompt should tell you more), look for a menu like "Boot Order", and change your setup so that USB devices come top of the boot list.