To make a bootable USB external drive for Windows Vista, you should first have Windows Vista or 7 running on your computer. Next, connect the external data device into your computer, either via the USB port on the side or a FireWire port. Before you begin, insert the USB device into your USB port and backup any data you've stored on the USB device. This will help you restore any unsaved data in case of damage to the external drive when formatting or reinstalling Windows.
If your Windows machine isn't too old, you may be able to install windows by using the Windows CD-ROM that came with your computer. Many people prefer to use the Microsoft windows installer instead of the bootable usb drive as it's easier to use and doesn't involve inserting or copying the actual disc. Bootable USB devices work very well in this case because they not only allow you to install windows easily. They also come with a built-in windows installer which allows you to download and install the operating system.
Many people prefer to use the Windows DVD-ROM to install windows. If your windows machine is too old to run the DVD-ROM, you can use the bootable usb drive that comes with the windows installation DVD. All you need to do is load up the drive, switch windows onto the drive, and follow the on screen prompts to install the operating system. If you're unable to boot up your machine, in most cases it's due to a missing driver. You can get your drivers from the manufacturer's website.