Error: The URL is not valid and cannot be loaded

Every time I go to any website, try to move between pages, etc., I get this message Alert: "The URL is not valid and cannot be loaded." But the interesting thing is, the page loads fine. I can do whatever I want when I navigate pages, but I have to click the alert off.

 

I'm assuming this is a Norton alert, but I'm not 100% sure.

 

Can anyone tell me what I'm doing wrong? Did I change a setting accidentally? It's driving me nuts!