This keeps popping up on my screen when on internet it stops me from visiting other pages. The URL is not valid and cannot be loaded.How do I fix it

First off how is your internet connection working? Do you try to load the sites more then once with the same results? Are you behind a firewall ?

Something is calling a web site that is no longer there. There are a lot of applications that you can install these days that "check in" for updates to themselves. Some of these things are viruses or other malware.

My first response is to wonder if you have an updated antivirus. Download AVG Free if you don't have AV on this computer and then Malwarebytes to scan your computer. Practice safe computing: backup, keep your computer up to date, install a reliable antivirus such as AVG Free.

I know I'm probably being paranoid but I hope I'm being over-worried.