I just tried to run irb for the first time on Windows and hit a minor snag.
I'm not sure whether this is something specific to /\ndy's one-click installer version, or whether it would be the same installing from source, but my irb.bat assumes that you are sitting on the drive where Ruby is installed. Hence, being on any other drive just generates an error message.
No big problem; I just added the drive to all of the paths mentioned in the .bat file.
I'm guessing that irb.bat is generated on the fly, because the rest of the path presumably depends on where you decide to install Ruby. So, the answer may be as simple as having whatever generates it include the drive letter in the paths.