Solved Unknown Security Issue

I have for several weeks been getting a message popup in a box on the web page I have displayed and a voice message is also played. The message in the box is as follows.
The page at Apache HTTP Server Test Page powered by CentOS says:
System Alert.
Your computer is infected with an adware or malware cusing you to see this popup
This may happen due to obsolete virus protestions
To fix call system support at 1-855-902-4667 immediately. Please ensure you do not restart your computer to prevent data loss
Possibility of data & identity theft, if not fixed

I have not been able to tie the message to a particular web page. When I click OK in the box the message goes away and then re-displays itself.

I am running windows vista on a HP elite book 8530P with Microsoft Security Essentials. The message comes up when I am using FireFox (update 45.0.2). I have not tried other browsers to see if the problem still happens. MS Security Essentials is using the latest virus definitions (1.217.1446.0). I have run the Security Essentials Full scan several times and it always shows no security issues. What is causing this popup??? I am baffled!!!

Many thanks in advance Eastbayboy

Sounds like Malware.
Run a scan with the free version of Malwarebytes.
https://www.malwarebytes.org/dl-confirm/
Make sure that you download the free version.

Further to Richards comment about free version,
before you finish the install you will be given the option to trial the pro version.The box is already ticked
Un-check this box or you will get nagged after the trial period has finished

Ran Malwarbytes and that seems to have fix the problem. No issues as yet.

Thanks, Eastbayboy