UNetbootin allows for the installation of various Linux/BSD distributions to a partition or USB drive, so it's no different from a standard install, only it doesn't need a CD. It can create a dual-boot install, or replace the existing OS entirely.

All it takes is a minor error in the Windows Registry or a virus infection, and your operating system can become unbootable. But with a properly configured USB flash drive on hand, you'll always have a compatible replacement no further away than your pock