Black highlighted box

1st time poster, so sorry if not in right section!
I have a problem with a black line box appearing in the active window that I type into, but then it stays on when I scroll down etc. I must have done something wrong somewhere, but I can't get rid of it. Its in every application I use on my mac, even on the desktop, if I click on an icon, a black box appears around it, instead of the normal just highlighting it.
Can anyone help?
Thanks, Loudle

Try going into System Prefs>> Appearance>> Highlight color

see if thats the problem.

I believe this is the section you were looking for: http://www.mac-forums.com/forums/showthread.php?t=44021&highlight=black+box

Command + F5 should turn it off

F5 and command did it, thanks to you both for responding. Cheers, Loudle