I got this same pop up from another website before.I'm not very IT literate so this might not make sense but I'll try to explain.
I clicked on the thing which said I had a virus.I was really worried and then something said I needed a programme called Personal Antivirus to remove it.
My computer then stopped booting up and I took it to a repair shop. The guy there told me the Personal Anti virus is itself and basically it'll ask you for money to install a programme to rid you of the trojan. There is no trojan, and if you pay the money it'll end up taking much more money from your account.
I don't know how you get rid of it without going to get it fixed so this isn't much help but don't pay out any money for the Personal Antivirus thing. I've now got avast virus scanner which (touch wood) is excellent and free :)