What is a possible situation when it might be necessary to boot Windows 7 from a USB device, a CD, or a DVD?

What is a possible situation when it might be necessary to boot Windows 7 from a USB device, a CD, or a DVD?

• to disable background services
• to delete registry keys
• to partition the hard drive
• to repair the system
  

  Explanation: Disabling background services, deleting registry keys, and partitioning the hard drive do not require rebooting a PC. It might be necessary to boot a computer from a USB drive in order to repair the system in some situations.