msvcrt-ruby18.dll not found when trying to use Heroku for Windows XP

I have a student developing on Windows XP, and he's getting a strange error when trying to run heroku keys:add. The error is:

This application has failed to start because msvcrt-ruby18.dll was not found. Re-installing the application may fix this problem.

After about half an hour of trying, my google-fu has failed me on this one.

Can anyone help?

FYI our solution was to install Ruby 1.8.7. Since Ruby has added significant performance benefits in 1.9.2, it's not a great solution, but then again, neither is developing web apps locally on Windows XP ;)