well, what kind of machine do you have? just holding down mousebuttons during boot gives you the early bootmenu from which you can select 'boot with no startup-sequence'. after that, you'll end up with a prompt on which you can type commands. you need to load the built-in text-editor called 'ed' followed by the file you want to edit. in this case, it's either the startup-sequence or the user-startup which are both placed in the sys:s/ folder. so type:
ed s:startup-sequence
or
ed s:user-startup
and put a ; before the line that loads miami. then close the screen which saves the file, and reboot.
