usb bootable windows 7 is windows 7 installation software booted via usb flashdisk. any flashdisk with capacity larger than 3.5 GB can be use as usb bootable windows. All you need to create usb bootable windows 7 installation is windows 7 installation software in ISO format, windows 7 USB DVD downloader for microsoft, and Flashdisk larger than 3.5 or 4 GB. you will need more than 4 GB for windows 7 64 bit installation. but for 32bit windows 7 installation you will need approximately 2.5 Gb.
here are the steps to make usb bootable windows 7 :
1. Make sure you have windows 7 installation in ISO format.
2. Download Windows 7 USB DVD Downloader form here
3. Run Windows 7 USB DVD Downloader, just choose OK to default install
4. After you finished the installation windows 7 usb dvd downloader icon will come up to your desktop. Run this program as administrator
5. Select the media and Click next
6. Open or Choose the Windows 7 Installation in ISO format
7. Choose Install and wait untill Finish