A bit of a late reply, but I was bitten by this error recently with Windows 7 64 bit so thought I would share my experience.
After checking through the config_check.rb file and trying to trace where it was failing, I eventually discovered that problem was with running the railsinstaller-x.y.z.exe as an administrator.
When you "Run as Administrator" on Windows, the drive mappings of the non-admin, logged-in user are not available to the cmd shell opened as administrator. For my case, HOMEDRIVE and HOMEPATH pointed to a networked share mapped to H:\
When the installer calls the config_check.rb script, is was being run under a different user account (one with admin privileges) that did not have a mapping for H: so the script fails.
As a workaround, after you see the cmd window with the error message, you can get the config_check.rb script to run by opening another cmd shell as the logged in, non-admin user, then running:
C:\RailsInstaller\Ruby2.1.0\bin\ruby.exe C:\RailsInstaller\scripts\config_check.rb
The script should run normally as it can now see the mapped drive.