How to stop installation?

Every time I restart my computer, I always get this notice saying, "Would you like to abort this download?" even though I'm not downloading anything. There is a yellow box symbol by the abort and continue buttons. I try to click abort, but every time I try to, the box goes away because my computer is restarting. How do I get rid of whatever is downloading on my computer?

I'm currently using a Macbook Yosemite, mid 2010 and I've only had it for about 5 months.

Thanks,

MDawg4531