This has been asked multiple times on the forum, with one being within the last week or so. What you see (or don't see) from control characters is based on what fonts you have installed and what language packs you have installed (language packs mainly affect WinXP). With mIRC being Unicode, the characters display based on what Unicode "considers" the correct output.
Khaled knows about some issues with control codes (such as ctrl-O and ctrl-I, which aren't visible, and is looking into possible options to make the characters visible without affecting anything else. It's just a matter of waiting for him to figure out a good solution. In the meantime, Ctrl-K's display is hardly a problem. It just looks different than you're used to.