Strange message appears when I visit Google

I've scoured the web for this problem and can't find an answer so any help would be very much appreciated.

Occasionally when I go to google.co.uk I get a message popping up which says "The Page at http://www.google.co.uk says:". It then has a message in what looks like Cyrillic characters with a white question mark in a blue circle. I get an "OK" and "Cancel" button.

Screen snapshot is attached.

This started happening a few days ago with no new installations of anything. It usually happens after opening my browser (I have google set as my homepage) but not always. Once it has happened it doesn't reoccur in the same browser session. I've tried disabling Firefox addons but to no avail.

System is W7
Browser Firefox

Anyone know what might be going on? Thanks.

 

Looks like a malware problem to me.

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