You shouldn't be directly deleting mirc.ini at all, you should be writing a new file and then using ReplaceFile() to move it over to mirc.ini.
mIRC already does what you are suggesting. It writes the output to a temporary file and if that succeeds, it then moves the original file to a separate temporary file, deletes that, and renames the updated file to mirc.ini. The reason for this is that a call to DeleteFile() can pend, which means that Windows will not perform it immediately, which can interfere with subsequent API calls. At least, this was an issue in Windows XP.