Please Note: Portable as in portableapps.com, not in the traditional sense of a language that can be used on multiple architectures or operating systems. Whoeve
In addition to the Lua suggestion, there is also Idle. It is basically a superset of Lua 5.1, with both the language (and libraries) and the implementation based on Lua. It was originally created to be a more complete scripting solution for Windows: because Lua is primarly intended for embedding, it has a rather small standard library and it is usually expected that the embedding application provides a rich library to Lua.
This makes sense for an embedded language, because, after all, there isn't much common functionality between, say Adobe Lightroom, Nginx and World of Warcraft, so there simply is nothing you can put in a standard library. But for a more general purpose OS scripting language, one would want a slightly larger library. Thus, Idle bundles a couple of libraries that are third-party (and sometimes hard to get to work on Windows) in Lua in its standard library.
Some of the things that the Idle standard library adds over Lua are tight Win32 integration, SQLite3 support, networking support, a PEG parser generator and archive support.
Also, Idle has support for embedding Perl and C code into your Idle programs.