There is a new installer in the Download section now. It's only ~200MB, tested, and works perfectly. It will start the batch file for you...you almost don't have to do anything.
If you are having issues with RS1.5a, this is now the only supported installer. Download it and reinstall
over a current install and it will fix any issues with the Mod itself. If it doesn't, then there is something wrong with your RTW install.