When developing for multiple ruby versions, such as 1.8.x and 1.9.x, it is convenient to have separate command prompts open, one for each ruby version. To do so for win32/64 platforms, create separate batch files for each ruby version installed and add something along the lines of
set Path=c:\ruby186\bin;%Path% cmd
This will open a new command prompt configured for the desired ruby version. Note, it does not matter if your Path environment variable contains additionally paths to ruby, since the desired ruby path is prepended to ensure precedence over whatever follows.