Thanks for the image.
If you could answer the following questions, that would really help in tracking it down:
Does this only happen when you open the Options dialog?
Does it disappear when you close the Options dialog?
Do you see it when you open other dialogs in mIRC?
If you install a clean copy of mIRC into your Documents folder, using the Portable option in the installer, and run mIRC from there, do you still see the issue?