Not all distros work with booting from a usb key. I have not had luck with
Mandriva for example. Most distros work fine for me. Make sure that you
choose ISOs that are the right architecture, 32-bit or 64-bit. Choose 32-bit
if in doubt. Also make sure that the key is empty (not necessary to format)
and mounted before you install the ISO. Try different sticks too because
they wear out and all are not equal.

I use Unetbootin on a regular basis and it works fine for me. Mint 9 is old.
It is the equivalent of Ubuntu 10.04, but I do not know how long Mint's
support runs or whether they have LTS. I admit ignorance so don't bother to
correct me. Although, I would be interested to know if they do have LTS or
not. I remember vaguely that they do but my memory is foggy about things
that I do not use on a regular basis. Whether they have LTS or not is
irrelevant because the version of Unetbootin in Mint 9 (even if most recent)
will be older than Mint 10 or succeeding releases. The idea with LTS is to
provide a stable platform and all applications are frozen in time (unless
you use backports).

Thanks for taken the time to help all of us.
From what you are telling us, Unettbooting won´t work with Puppy-Linuxś.
Unfortunatelly, I don´t have right now all the facts, I am away from my office
and using another computer.
What do you suggest I should do?? What utility to use to install Puppy-Linux
Oops, I just realized, I think, I downloaded the wrong version of Puppy, maybe I
should have dled the Mandriva one.
I will look into it as soon I get to my office.

Puppy Linux can be installed but works best running from a USB or disk.
Note: this is different from installing it to usb.I do not like Puppy as a
main OS but some do. It is sparse in terms of what it offers for my taste.
It is meant for occasions when you cannot run a full desktop. Some will
argue that point, but when you can run something like Ubuntu, why would you
want less? It is stripped down to run fast, but it is also not a full
experience. There are trade-offs.

Note: Puppy is NOT based on Ubuntu. (You are asking this in an Ubuntu
group.) There was an Ubuntu based version last spring, but Puppy is its own
distro from scratch. There are different versions kicking around. Slacko the
new one is based on Slackware and Wary is their LTS release, just out.

For installing to usb see: pendrivelinux.com

Running from an ISO is what unetbootin is intended. This is not writable. It
is just like a live CD or DVD. It is read only. You cannot install
applications or save data.

Pendrivelinux.com makes a persistent disk that allows you to boot from the
disk, install and save data. It is like a computer in your pocket.

See http://www.pendrivelinux.com/puppy-linux-on-usb/

This of course depends on your intentions and your needs.

Actually Puppy Linux 5.2.8 Lupu is based on Ubuntu Lucid. The newest Puppy was
released this week and it is called Slacko, based on Slackware.
You are right that it is somewhat sparse, but it is light and fast and good
for old hardware.