Strange yellow box on desktop

Virus scan?

What, and from where?

Try deleting the virus scan app. You don't need it. That may be the cause of the yellow block.

Forgive the candor, this is garbage. There is no polite way to say that.

Get rid of them.

I do not know... but there is no point trying to solve that until your computer has been cleaned up. So my apporach would be to get rid of the known problems, and see if that alone solves the other issue. If it does not, we can deal with that separately.

MacKeeper is scareware, marketed extremely aggressively by an unethical company. It has been known to damage systems to the point that they have to be reinstalled. You need to remove it right away.

iAntivirus won't hurt you, but it won't do much to help you either. It's not very good at detecting Mac malware.

Regarding the yellow box, that's not a symptom of any known malware. It's very unlikely that it would be a malware issue anyway.

It's hard to say what it is, though. Start by testing in safe mode... does the box still show up when you are in safe mode? If so, it's most likely something wrong with your system or a hardware issue. If not, and if it shows back up when you reboot normally, then it's caused by some third-party software you have installed.