Thanks for your bug report. These are helper windows that mIRC is using for the Aero MDI theme. Technically, it should not be possible for them to be visible or to appear in the Windows taskbar.
Are they visible the moment you start mIRC? Or do they become visible after a while?
They appear after awhile, not on startup.
Are you running any scripts, addons, DLLs, etc.?
No, it's a purely stock non-portable install.
If you install a clean copy of mIRC, using the mIRC installer, with the "Portable" option enabled in the installer, into a new, empty folder in your Windows Documents folder, and run mIRC there, do you see the same issue?
I'll give that a shot and report back.