The operating system that a computer runs is determined by the hard drive from which the computer boots. If you have a Windows 7 installation on an external USB hard drive, you can run Windows 7 by adjusting the computer's Basic Input Output System, or BIOS, setting to boot from the external USB hard drive. When the computer boots from the external hard drive, it runs Windows 7, regardless of the operating system installed on the main hard drive of the computer.
Connect one end of a USB cable to the external hard drive. Connect the other end of the USB cable to the USB port on the computer.
Shut down and power off the computer. Turn the computer on. Watch the screen for a message that offers the ability to enter set-up by pressing a key, usually the "F2" or "Del" key. For example, it might say "Press F2 to enter Setup." Press the key when the message appears.
Review the menu selections and options within the menus on the set-up screen. Find a setting labelled "Boot Order," "Boot Options" or something similar. Select that option.
Review the list of devices in the Boot Order list to locate the external drive. Keep in mind that, instead of "USB Device," the drive might be referred to as "Removable Device," "External Device" or something similar. Refer to the instructions on the screen for the keys to use to reorder the devices in the Boot Order list. Use the keys indicated to move the USB external hard drive to the first place in the list of boot devices.
Refer to the instructions on the screen for the keys to use to save your changes and save your boot order changes. For example, you might press "F10," select "Save" and press "Enter." The computer reboots automatically and runs Windows 7 from the external USB hard drive.