it's simply the font you are using.
change it to something, i reccomend verdana. my favorite non-fixed font. if you must have a fixed font (often the case with me...) then i suggest either lucida console, or fixedsys.
btw... certain characters are seen for colors in mirc, and are automatically purged in order to provide for colors. if you happen to be using any of these "special" characters, they may totally disappear because of it.