Regarding Toby's patch:

Probably _is_windows_nt_internal_command() isn't needed or desired. It's more of a command-line parsing issue, rather than a file-search issue. For example, CMD will search for an internal name if it's quoted in double quotes in the command line, such as `"dir" spam`, and where.exe doesn't reserve the names of CMD's internal functions. If it's kept, note that the following four names are missing from the nt_internal_commands list: 'BREAK', 'MKLINK', 'VERIFY', 'VOL'.

The patch also makes a couple incorrect assumptions, corrected as follows:

    * A filename with an extension will be found and executed 
      even if the extension is not in PATHEXT.
    * The PATHEXT extensions are tried even if a name already
      has an extension.