I've found no easy way to add directory paths to the system's PATH environment variable, so I created AddPath-1.0-Win32.zip. Here is the readme.txt file describing its usage:

AddPath 1.0 - Jan  6 2006 - 10:18:11
Copyright (c) 2005-2006, Ross Smith. All Rights Reserved

Usage: AddPath [options] path [path2 ...]

Options:
-v VarName Append path(s) to environmental variable VarName (default is Path)
-s         Modify variable in the system environment (default)
-u         Modify variable in the current user's environment
-d         Modify variable in the default user environment
-e char    Change % substitution character to 'char' (default is ~)
-n         Don't add path if the variable already contains it
-p         Prepend path(s) to variable
-r         Replace the value (default is to append)
-x         Use REG_EXPAND_SZ type (default if ~ char is used)
-z         Use REG_SZ type (default)
-c         Show version and copyright statement
-?         Show this help screen

Examples:
  rem append c:\bin to the system Path variable:
  AddPath -n c:\bin

  rem prepend c:\usr\local\bin and c:\bin to the system Path:
  AddPath -n -p c:\usr\local\bin c:\bin 

  rem Add "C:\Program Files\Java\bin" to the system Path:
  AddPath -n "C:\Program Files\Java\bin"

or, even better:

  rem add or replace JAVA_HOME with %ProgramFiles%\Java
  AddPath -n -r -x -v JAVA_HOME ~ProgramFiles~\Java
  
  rem Append %JAVA_HOME%\bin to the system Path:
  AddPath -n ~JAVA_HOME~\bin

Other keywords: setx, setenv