Replying to Pop-up saying I won then redirected to spam website.

I had just loaded 'my threads' and I get a pop-up saying 'You are our 1000th visitor you've won' with no exit button just 'okay' or 'cancel'. I clicked cancel and it re-directed to http://www.free-choice.com.au/cgi-bin/wingame.pl?partner_pk=419&wingame_pk=42&sub_id_postback=7c3a83526a9e0a53947472f19731b195&sub_id=CD14387

I don't know if it's from NS but it only happened when I was on NS so I thought I should post it up.