When installing the binary on windows, you need to make sure that you have
Tcl/Tk 8.3 installed.  You can get it at scriptics.com or activestate.com.
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 installation of the cygwin dll.