What USB creator are you referring to? The general process is to download the Ubuntu (or any Linux) iso file and either burn it as an image to a CD/DVD or use software specifically designed to create a bootable flash drive. Examples are unetbootin or pendrivelinux.
What user/password screen are you referring to? Is this to log in to vista? That's a totally different problem than booting from a usb/flash drive. More details on exactly what you have done and specific problems you encountered would help someone to help you.
You mention Ubuntu in your thread title, does that mean you also have it installed? You can download a Mint iso from their web site.