Can you post a screenshot of the popup you are seeing?

 

That will likely be difficult. Since I posted the first question, I ran Malwarebytes, quarantined some "non malware", and restarted. Before the desktop loaded, I got a blue screen for about 1-2 minutes, then the desktop. but no notice of invalid windows.

On opening firefox, I got the message that the "proxy server is refusing connections" .

None of these thing happened two days ago.

Also , The "Cntl Print screen" function appears not to work.

 

I'd suspect something is on your machine that shouldn't be.

Please read all of the following instructions found here: http://www.techsupportforum.com/for.../new-instructions-read-this-before-posting-for-malware-removal-help-305963.html

After reading all of the instructions found above post the required logs in a new thread: Virus/Trojan/Spyware Help

Please note that the virus team is very busy and will get to you in due time. If you do not get a reply within 72 hours then you may bump the post.

Do not post any logs here!

 

I suspected....
Will do.