That's weird. I have no idea what's causing that. When did you install Arena, and has it always had this problem? Did you try rebooting the PC?
I don't know, maybe check all of your Arena settings again? I guess a reinstall of Arena would be a last resort. (Back up all of your important Arena files first.)
I have a problem with the appearance of arena chess. Some colour schemes look weird on my computer. Note the green squares here for example: