For most Windows users, the regular installer from the "Windows 2000 / XP / Vista / 7" link is the best and
simplest option. If you've installed it already, run it and click "Check for Updates..."
in the File menu to download the new version, if there is one.
If you want to run the program without installing it, there are some other options. The
"Portable .Zip File" version has all the files from the installer, but does not install
them for you. The "Application Only" file is the same as the Portable one, but does not include any simulations.
(There's no demo version of this because, without the demo-runnable simulations, you couldn't do very much with it.)
all files created or edited by Pixel Gravity will be in a folder called
"Pixel Gravity" in your "Documents" folder (or "My Documents" in
older Windows versions). To make it save everything in the same folder
as the application (.exe file), either turn on "Portable Mode" within
the program--go to General->Program Settings in the menu--or
just delete the file "NonPortableMode.txt" from the application folder.
This allows it to run off a thumbdrive, and is helpful for attempting
to run it on non-Windows machines that do not have a Documents
Pixel Gravity is designed for Windows,
but should run decently on Mac and Linux with help from emulators or similar programs. It's been tested and
seems to work on Linux with Wine. For Macs, read this.