IIRC this is a windows bug for MDI child windows using the aero interface. I reported this a while ago, but this was either solved by microsoft in a software update or perhaps Khaled made a workaround because it does not happen for me anymore. I do remember reinstalling vista between the time I last had the problem, though, so the bug could also be related to specific user settings.
The thing is, this should happen to all your MDI applications, not just mIRC.
