Use setx to permanently set an environment variable
If you’ve used windows environment variables at all, you’ve probably wished you could set them in a command window for use in another program.
You can’t change the environment of an already-running program – the environment is copied when a program starts – but you can set environments values for your user session that will be picked up by any new programs you launch with setx:
setx myvariable thenewvalue
Much easier than right-clicking to get to My Computer properties.
setx /? will also tell you about setting permanent, machine-wide and remote machine environment values.
setx comes with Windows 7 or in the Windows Resouce Kit.
How to unset a variable set with setx
setx myvariable ""
See http://support.microsoft.com/kb/195050 for more details.