go to the folder you installed mirc in and look for the MIRC.EXE file, and run that, its possable something is deleteing the file, If it does this the link/icon for it wont work since that simply runs the mirc.exe file, (which if missing wont run). The reason it might be missing could be either something evil on ya pc, like a virus, or something "so calledly" not evil, like an anti virus.