addpath.exe

Usage: addpath <path> (download)

Modifies a variable in the registry (HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment\Path) to include your path.

Any program that's already running won't see the change, so you'd have to restart it.

Other path modifiers are either hard to use (they come with gazillion options so you actually have to study them for longer than it takes to do this with the Windows UI), or fuck up the key by not expanding %SYSTEMDIR% substrings in it and storing the result as REG_SZ. Also they don't check for duplicates and happily add your path to the search path multiple times.