Good advice above: the main thing is to get the BIOS set to make the USB drive the first priority for booting. BUT be aware that certain brands of thumb drive - e.g. Kingston DataTraveler - will NOT boot an operating system even if the computer's BIOS is set correctly as the priority. Kingston mentions this on its website, but it completely confused me until I found it. I reformatted etc. etc. etc. to try to make it work and it just won't. By comparison other drives - e.g. SanDisk - work fine.
Mint 16 XFCE on Dell laptop, replacing XP, and dual booting W7 and the same Mint on an Acer netbook. Test driving Mint 17 Cinnamon via Persistent Live USB on these plus a W7 Lenovo desktop with peripherals.