UPDATE: I have checked this laptop thoroughly from top to toe. No trojans, infections, nothing, checked for rootkits, the lot. All clear. Still have the occasional pop up only when accessing Deviantart - no other site.
Last one was nasty - responding to one of pendragon's comments an advert tab was opened instead of the DA page and a page purporting to be from Adobe complete with a supposed flashplayer download. It stated first "to view this page you need to upgrade Flash".
Now, I never install anything unless I really need to, especially Flash.
I checked the tab. The URL had the word "flashplayer" misspelt as "flashpalyer" and the tab was named ad0be instead of adobe. It was a trojan advert attempting to get me to install a dodgy Adobe most probably complete with a virus.
I am pretty sure this stuff was generated by an advert shown by Deviantart. It only appears when I am in Deviantart. The laptop is clean! I don't know what to say.
So, I've added Avast browser security to my browser but I'll probably only enable it when I am checking DA - and we'll see what it comes up with.
I've done a quick search and it seems I am NOT the only one who is having these pop-ups - plenty more are.
Type "deviantart dodgy pop ups" into google and see what is displayed. The fact is that DeviantART serves up dodgy ads and malware links. TAKE CARE.