Download the latest self-installing Windows executable here. Works on both 32-bit and 64-bit Windows 7.
A self-installing application will be coming soon. (If you'd like to contribute to building the app, contact kale @ erwinjr.org.) For now, you can install from source by following the instructions located in the wiki.
Here is a snapshot of the ErwinJr 2.1 source code as of 13 March 2012.