Thanks for the feedback.
I am afraid I am not sure what else to suggest. I tested mIRC on seven different Windows XP, 8.1, and 10 installations and could not reproduce the issue.
If I Google
this issue, there are quite a few similar reports such as here
, and here
. Unfortunately, there do not seem to be any obvious solutions, eg. it could be due to a corrupted installation of DirectX, in which case you could run "dxdiag" to see if any errors are reported and/or reinstall DirectX or your graphics drivers. You could try shutting down all background applications first eg. MSI Afterburner, since they might be interferring. Some reports indicate that disabling anti-virus software resolved the issue. You could also try running the System File Checker
If you do find a solution, please let us know.