Boot.ini resides in the root directory of you first or only partition. It
normally has the hidden and system attributes set. You won't be able to
see it in Explorer unless you set it to show system files, which are not
displayed by default. In Explorer, go to Tools > Folder Options > View
and uncheck "Hide protected operating system files".
If you don't want to do that, you can edit boot.ini by opening a command
prompt, changing to the appropriate drive if necessary, and typing:
attrib -s -h -r boot.ini
Then start notepad: notepad boot.ini
When you're finished and have saved your changes, restore the essential
attributes:
attrib +s +h boot.ini