From the Ubuntu link in post#7:
If you're not sure what this means, the computer can boot from various devices. It's usually the hard drive where Windows is stored, but for your situation you need to change the setting in BIOS so the computer boots from the new Ubuntu USB stick.
To enter BIOS, keep tapping the appropriate key straight after switching the computer on. It's a different key for some computers, but it's usually F1, F2, F10, DEL or ESC. (See
here for instructions). It should say on the screen which key to use, or you can check in your motherboard manual. Insert the USB stick in the infected computer, then go to the screen where the boot order options are listed, change it to 'USB First', hit F12 to save the settings and exit. The computer will now bypass the Windows hard drive and boot into the Ubuntu USB stick.