Like s00p, I think you're probably talking about an "acromancer" script -- a script that will automatically convert certain combinations of characters into something else... for example, letting you type wtg and it converts to way to go. Such scripts can also add colors and formatting or completely change what you type. If you don't want that, either get rid of the script or disable that part of it (if it can be disabled). mIRC itself will not convert what you say without having a script that does it, so a clean install of mIRC from this site will solve this for you if you don't know how to remove the script.
