I am still unable to reproduce this issue here. Here is what I am doing:

1) Close down all other software that is running, including other copies of mIRC.
1) Copy mirc.exe to a new empty folder with no scripts or dlls.
2) Run mirc.exe by double-clicking on it.
3) Maximize the status window.
4) Go to the View menu and enable the Treebar.

If you perform the above steps exactly, are you able to reproduce the issue?

If the above steps don't reproduce the issue for you, can you describe step by step how you reproduce it?

Also, which version of Windows are you using? Are you using XP themes? Are you running any other background software that might be affecting mIRC?