edit the file GLOGIN.SQL in the %ORACLE_HOME%\PLUS33 directory.
add the following line:
DEFINE_EDITOR=<path and name of the editor of your choice
next time you start SQL*Plus, this will be your default editor.
DEFINE_EDITOR= D:\Program Files\UltraEdit\UEDIT32.EXE
--- It's Wrong
DEFINE_EDITOR="D:\Program Files\UltraEdit\UEDIT32.EXE"
--- It's Right
