Planets is a simple interactive orbital simulator.
Before running planets, you need to make sure that you have Tcl/Tk 8.3 installed. You can get it here:
http://downloads.activestate.com/ActiveTcl/Windows/8.3.4/ActiveTcl8.3.4.1-9.win32-ix86.exe
Once it's installed, you may need to reboot, to ensure that the path to the Tcl libraries (typically C:\Tcl) is in your path. You can add it manually if you like.
Note that you also need the cygwin dll, which is included in the zip file. If you already have cygwin installed, then you can run planets from the cygwin command line without this dll.
Take a look at getting_started.html to get a sense of how to use the program, and look at KEYBINDINGS.txt to see the full set of keybindings.