I very much agree with gogoud's answer, and this is just expanding on it (too long to comment).
I am consodering a change from win7 to any linux distro
Good move. I did the same a few months ago and didn't look back.
I want to know whether I can change the envirnonment by downloading
them to a usb stick from another computer and installing it in my pc
(...) is it possible to download softwares to removable media and
install them to other pcs?
It sure is. For Linux Mint, you can download packages in
.deb format here: http://packages.linuxmint.com/
Each distribution has a command-line syntax for installing locally-available packages. I'm not familiar with Mint, but in Arch Linux the package manager is called
pacman (yeah), so you would use the
pacman -S <package_name> syntax to install from the online repository or
pacman -U <package_file_name> to install from a local disk or USB drive. Using the Cinnamon package for illustrating:
pacman -S cinnamon
for online installing, and
pacman -U cinnamon-3.2.8-1-x86_64.pkg.tar.xz
to install from the
cinnamon-3.2.8-1-x86_64.pkg.tar.xz file you have downloaded elsewhere - and you may prefer to specify the full directory path instead.
Bear in mind that Desktop Environments are very complex and usually not contained in a single standalone package. If you have a look at the Arch Repository entry you'll find that Cinnamon has a number of dependencies, and some of those are unlikely to be present in the system so you'll have to install them manually beforehand (except if using
pacman -S which deals with everything for you). For Cinnamon specifically, they would be:
nemo - at the very least.
Linux Mint is sure easier to set up and get going, and if you choose this distro, you should know (and already do, probably) that they offer different "flavors" when downloading from the website, each integrating a different Desktop Environment. The Cinnamon and MATE editions are more "traditional", while the newer ones sure have their niches.
Now that you know that switching between DE's won't be exactly easy - the Mint team puts a good effort in testing their integration and it's quite non-trivial-, if possible, do practice on a Virtual Machine before you make your choice.