Well, it's really strange. I was thinking about it for some time, but I don't see any difference between the About dialog and any other avast! dialogs. Additionally, I don't see any difference between the first opening and the subsequent ones.
So, I have really no idea what might be causing this.
Try to delete the value AboutRect from
HKEY_CURRENT_USER\Software\ALWIL Software\Avast\4.0