I don't want Windows anymore
You better be sure about this!
I don't know which computer you have but there should have been a technique which allows the user to interrupt the first booting process as soon as the computer is turned on. Usually this is achieved by pressing the
ESC
key. For example on an ASUS computer I have, as soon as the screen makes any sign that it's coming on I press Escape. On other computers this key could be different, such as
F12
. You will have to investigate this on your computer.
Another thing that is potential PITA is to find out how to get into the BIOS-like program which allows the user to choose an EFI file to boot from. This is very important, because these days almost all Linux live systems boot via UEFI. If this screen is not accessed, from how you were, you will be stuck always going into Windows. Alternatively, in the BIOS-like setup it should be possible to change the order of the devices the computer boots from. The "external USB" should be first on the list, followed by "internal disk" and lastly any other removable media such as CD/DVD, or networking option since you are using a pluggable USB to try to boot.
I'm saying this stuff because I don't know about a bunch of other computers but have two Hewlett-Packard laptops. In either of these I press
ESC
as soon as the screen lightens up from turned off. Then I get a menu which tells me if it should go ahead to the BIOS-like settings or proceed to boot into Windows. On either of these computers I press
F9
and then I get to choose which EFI file to boot from. Sometimes on one of my computers I have to hunt it down, because I have installations on external media besides three more on the groaning internal hard disk (not SSD).
The trick is to boot into the live mode of Linux Mint and then you could use the installer and just tell it to use the entire disk all for Mint.