You can probably do an online installation direct to USB if everything works as it should... but if anything happened to your windows hard drive, you can kiss it goodbye, because the initial process repartitions the drive, and if it picks the wrong drive to partition (or you do), windows will be gone.
So I always burn the OS image onto a DVD for installation so I can disconnect the hard drive before installing the OS onto a USB drive.
It takes longer, I hate to take the computer apart, but it saves a week of rebuilding an old windows system from scratch.
Linux Mint works better than windows for most things. You will find the applications as good or better than for windows, and it is much simpler to update. And if you have to reinstall it because an old hard drive failed, you don't need permission from Microsoft. That alone is a fantastic reason to switch. You never have to try to deal with Microsoft again.