The most likely reason is that the channel is using unicode for the channel name or topic and since mIRC is not a unicode application and needs to use a custom display routine for the titlebar, Windows Vista/7 disables aero glass for that window. I am currently working on converting mIRC to a full unicode application. It is an enormously difficult project, the largest that I've undertaken with mIRC so far, but hopefully, if all goes well, the new version may be available before the end of the year - at the very least as a public beta release :-)